I have to make a smash cake for a lactose intolerant baby. How would I calculate the percent of cashew milk to reg. milk to get the result that regular milk would give? If that makes sense.

I would just measure the same amount.  Unless the substitute  liquid was considerably thicker or thinner.  I imagine cashew milk to be similar to regular milk.

What about fat percentage?

Hmm, now that I think about it were you instructed to use cashew milk?  Has the baby been tested for nut allergies?  Might be safer to start with something more benign like coconut or rice milk.  As for the fat content I doubt the baby will care, but if you have to make more lactose free cakes (don't forget the butter) it would be worthwhile to google it and figure out the difference between cashew and whole milk.  If it comes up short you can add some oil.

you could probably use water instead of milk in this case.  Or soy milk is an option too.  

@Brookebakescake  bakescake wouldn't the water take away from the flavor? @Pastrybaglady   They baby drinks cashew milk he really likes it from what I'm told.

I only use Almond milk in my cakes in place of Cow's milk....just a simple replacement. Never an issue. Don't add any  extra fat. When I did use Cow's milk, I only ever used skim milk. 

Does it taste any different? @jchuck  

My personal experience with different levels of fat in cake is that it's not a taste difference it's texture.  I used to use 2% all the time because that was what I had on hand and my cakes were very good, but then one day  I decided to try bumping up the fat by adding a little half and half to it and man, it made the cake melt in your mouth soft!

Okay so lets say that the cashew milk is 50% fat and the milk is 75% (I don't know). How would you make the cashew milk go to 75% without adding dairy to it?

No. I've used Rice milk, Hemp milk. Doesn't affect the taste, especially if your adding an extract/flavouring. Pastrybaglady is correct, the fat in your liquid does change the texture. If you're little one can tolerate soy, you could add a soy based yogurt or pudding. That will make a dense type pound cake. Or a 1/4 cup if shortening added will also make a melt in your mouth cake. But many shortenings contain trace amounts of soy. So again, make sure your little one can have soy.

Thank you that was very much appreciated.

Hmmm... I looked up whole milk - 8 g. fat; cashew milk - 2g. fat.  So it's closer to 1% milk.  But again, the baby is not going to care!

Do you think shortening would do the job to make up for texture?

Do a trial run, if you can halve or even quarter the recipe. Unless the Mom is one of those control freaks, she's not gonna care about a slight (probably not even noticeable) texture difference. And would you normally use butter? If so, shortening might be a good substitute, but do check with the mom on any other issues (soy, coconut, etc). My little one is only a little bit lactose intolerant, so we use a brand of milk called Lactaid. He can tolerate butter, but regular milk is a no-no. I've also used canned coconut milk (full fat with cream on the top, mmmm) and I haven't seen a difference in my muffins.

And if you're worried about the baby with cake texture, he's not going to care unless he has a sensory issue. In that case he will just barely stick his thumb in it and slowly explore it instead of digging in (which is what my kiddo did/does).

A test run is always best in case there are any last minute issues! :)

I just want everything to go smoothly they really don't think I can even do it. Which is such criticism to me. I've proven that almost every time someone has told I couldn't do something I did it. I just want to blow the roof off with this smash cake.

No, it's not gonna make a huge taste difference, especially in a smash cake.

I would use oil instead of shortening to replace any fat, if you decide to go that route.

Coconut oil? He doesn't have nut allergies.

Olive oil is a good option instead of shortening, healthier for sure. Only oil that you have to worry about is peanut oil.  The rest, olive, canola,vegatable, sunflower, safflower oils don't contain any nuts.

Have you tried olive oil.?

Yes, I have. Any oil works well. Very experienced baker here. Baking for over 40 years. 

Wow.! I'm definitely going to try it thank you. I wanna get there one day.

Ha Ha...one cake, brownie and lof of bread at a time!! Enjoy the journey...

Why not just use canola or vegetable oil? Don't stress yourself out over this; I think you're overthinking it.  Just breathe! You'll do fine!

I want to test a variety of things to get the best outcome. I tend to overthink things all the time. Its something I really have to work on.

Perfecting your recipes is a wise thing to do.  Once you find something that works for you you'll be more comfortable and the whole process will be less stressful!

I know I was so stressed and overwhelmed yesterday.