So as promised, here is a tutorial on how I do petit fours. I know I'm a freak for enjoying making these, but maybe because I've made a million, have my system & technique and they come out perfect every time. 
 
First please read all of my steps to absorb it all.   Maybe read it again if you are the slightest confused. Then go buy a cheap Sarah Lee pound cake to practice with. 
I usually do 2 dozen at a time of all the same flavor.  I had to make 3 flavors this time which is not typical..Lemon, Regular & Chocolate. Here is the final product.

I use the typical petit four recipe you find online.  1-2lb bag of powdered sugar, 1/2 cup water, 1/2 cup karo syurp.  I use 2 tsp of almond extract where most recipes only ask for one.   Follow the directions on the recipe.   Keep PT4 icing covered at all times so it does not set up/form a crust.  I think I can get 3 dozen petit fours from one batch.  You can store left over icing in refrigerator and reheat for next time.

Step 1.

I bake a one-box WASC and use the 9x13 half sheet pan.  This will make the perfect height and I won't have to cut off a ton.  I set my Wilton cake saw on the 3rd line and saw off the hump.  Trim off enough top so you have no more than a 1 1/2 tall cake.   You do not want petit four towers.   If you have too tall of a petit four, it will fall over when you do the pour. 

2. I put a thin layer of icing on top my cakes.   Petit Four icing is very sugary and I think this just makes it taste so much better.  As you can see, I have mine split up because I had to make 3 different flavors.   Just cover the entire top of your quarter sheet with icing.

3.  Cover with waxed paper.   You can use your hand or a smoother to make the top all smooth and flat.   I put in frig overnight. The cake must get cold.  Do not rush this step or you will have a crumbly mess. 

This is my knife of choice.   It is a filet knife.   I like it because the blade is so long and the most important reason is that it bends.   More about that later. 

4.  The next day cut off the 4 edges of the cake.  I dip my knife in hot water after each cut. 

This is just a picture of the trimmed sides

You will find petit fours come in many sizes.   I have seen little 1x1 bite size but I don't like that size.    You will need find a ruler of the size of petit four you decide to make.  I prefer about 1 1/2 x 1 1/2.

5.   Next lay your ruler on your cake and make your vertical mark.   Don't cut yet.   If your ruler sticks after you remove it, just take a wet offset and smooth it out again. 

6.   Next make your horizontal line.   Do not cut yet. 

7.  So here are my scored lines.  Your cake will not line out perfectly.   You will have an edge (or two) that needs trimmed.  You can see the left side needs trimmed and the blocks closest to the top of the page are not 1 1/2  x 1 1/2.   Those 2 sides need trimmed.

8.  Again, with a wet knife get ready for the cut.  (Can you see the turkey in the background  watching! LOL...my husband is a hunter)

9.   With a hot/wet knife slice the edges that are not needed.   Take a snack break at this time if needed.

10.  Dip the knife in hot water and make your vertical cut.  Do not saw.  Try to do it in one motion.   That's one of the reasons a long bladed knife is good. 

11.  Dip the knife in the hot water and make your vertical cut.   Again, I would normally be cutting a half sheet.

12.   Put the square little cakes on a grid inside of a shallow pan.   I normally line up 4 across and 6 down to do 24.

13.  This is my favorite pouring container.  It's light weight and I think it holds an entire batch of the PT4 icing.  It's a walmart measuring cup that lost all of its markings so it is now the PT4 cup.    Put your ready made PT4 icing in it and zap in the microwave.   If using left over, you will need to zap at 1 minute intervals.   If it is fresh..still warm, then zap at 30 second intervals.    The consistency is VERY important.

You need it to be thin..but don't boil it in the microwave.  Thin icing makes coverage so much easier.   Do not ever add water to make it thin.  This will bread down the icing and now give you a "shell".   You get it thin through microwaving.   This is where some practice will need to happen.  The more you pour...the more you will just know how thin you need it to be. 

14.  Ok..I could not take a picture while I was pouring.   So I am going to try to explain my technique.

Starting on the far side, with the container close to the cake, I make vertical zig zag motions pouring beyond the size of the cake..just zig zag from one side to the next.

Then I do the same thing horizontally..zig zag beyond the size of the cake (to ensure the sides get covererd).

Then I do one swoop around the outer edge.

This is done slowly....but quickly.  You need to get coverage and you cant see all four sides while you are doing this.   That's why you have to zig zag beyond the cake so you know it's hitting the sides.

Gosh I hope this makes sense.    This is where you will need  the practice.    Getting 100% of all 4 sides covered takes practice.   But once you get your pattern and rhythm, you will zoom through it.

15.   You will only get about 6 covered at at time before you run out of icing.   Remove the rack, and scrape up the icing.   Pull out any big crumbs if you see any and put the icing back in the pouring cup.  Reheat on 30 second zaps and do again until all your little cakes are covered.

16.   Let the dry.  Do not try to remove too soon.   They are covered..they are protected...do something else.  I have let them sit for an hour before while doing another cake.  

Then take a sharp knife and just cut around the edges to release the excess icing from the cake.  Do not remove yet...just slice around each side.

17.   Removal.   This is where I LOVE the filet knife.   I lay it as flat as possible and slide it under to release.   If you do not have a filet knife, use your longest, thinnest offset spatula.