I have been using a doctored cake mix recipie that I saw on a post a while back. Cake mix, milk(instead of water), one stick melted butter(instead of oil), a small box of pudding and an extra egg. I love the taste of the cakes, but they often sink in the middle. I'm wondering if there is too much liquid, or am I not baking them long enough, they seem done but somethings wrong. Any thoughts??

it sounds like you just aren't leaving it in long enough. I only go by time for basics, I always test by sticking a wooden skewer into the middle. When it comes out clean, I know it's done. (sorry that makes it sound like I'm telling you you don't know how to do this, but it's how I do things.)

Well, it is a normal reaction in some cakes, for the middle to sink a bit either during the cooking process or while cooling. As long as the cake tested clean when inserting a toothpick in the centre, the cake should be done.
There are many reasons for a cake sinking in the centre, sometimes the fat content in relationship to other ingredients is too high. Sometimes the cake is overbeaten.
The only cake I would exchange butter for oil used is an Extendacake recipe I have where I know it works out fine. And if a recipe calls for it. Some of the newer cakemixes are geared towards adding butter and this should work out fine. But generally butter can just be too rich for some cakes and cake mixes where the other ingredients are not geared towards butter. This may be your problem.
Butter generally makes for a more dense textured cake.
Sometimes a cake that rises rapidly due to the leavenings, does this because we grease the sides of the pan unlike commercial bakers who normally leaves the sides ungreased to allow the cake to support itself on the sides as it rises.
Here is the recipe I add butter to:
Auzzi From the Wilton Site's Extendacake Pound Cake
1 pkg of any kind of cake mix to which you add all of the ingredients called for on the box
In addition:
1 cup all-purpose flour
3/4 cup sugar
1 tsp. baking powder
1/2 cup Crisco oil - she substitutes 1/2 cup softened butter instead and so did I - wonderful!
2/3 cup water
I beat the butter, then added all of the ingredients the cake mix called for and then all of the other ingredients. Then I blended on low for 1 minute, then 2 minutes on medium, scraping the bowl down. Gnerally, cook at 325F for the larger cakes, 350 for smaller and you will need to add baking time, perhaps about 10-15 minutes to the times given on the cake mix box. I checked ever 5 minutes or so over the cake mix times.
Hugs Squirrelly Cakes

Maybe too much fat? When I use a doctored mix, I substitute equally for the oil - 1/3c of butter. Maybe the entire stick is too much? Just thinking out loud...

Thank you so much squirrellycakes for the recipe, I can't wait to try it. I know the cakes are done, it must be the fat content ratio.

I will also try less butter 1/3 cup instead od the whole stick, that thoght had crossed mt mind as well. Thanks Ironbaker.

If you live in a high altitude place this will make your cakes sink as well. Anything over and around 2000 feet above sea level will have this problem.

Good luck
Dana

Another reason why your cakes might be sinking..
Don't open the oven door till after 30 minutes of baking (for regular cakes). Your cake might not be cooking evenly too. Also, DON'T LET ANYONE WALK IN THE KITCHEN, SLAM DOORS!!! Just Joking!! You'll figure it out! Good luck~