Yesterday, I baked up a half batch of the wasc using butter and no other substitutions. The cupcakes were light and fluffy 20 minutes out of the oven (what can I say, I couldn't wait to taste it

) but today the cupcakes are very dense and dry. I love the taste but how can I ensure it maintains that fluffy texture?
This is how I halved the recipe:
3.5 c. cake flour (I use unbleached kaf,

I hope this isn't the problem)
1 2/3 c. sugar
1TBS+ 1/2 tsp baking powder
1/4 tsp baking soda
1 tsp salt
2/3 c. butter (softened, room temp)
8 oz full fat sour cream
1/2 c. milk (I used about 3/4 c. because the batter looked too thick)
1/2 TBS vanilla extract
3 whole eggs
FYI: I sifted the dry ingredients into my mixer, mixed it for a bit and added the butter, and added the liquids (sour cream, vanilla, eggs, and the milk last). I mixed it for 2 minutes on 5 (on my KA). I baked the cupcakes for about 20 minutes at 335F. I set out to use only 1/2 c. of milk but ended up using almost 3/4 cup for the half batch.
Do you have any suggestions for what I might have done wrong? Should I have dumped all of the ingredients in as stated in OP? My apologies for so many questions but I am fairly new to this (I have always baked from scratch but I want to learn as much as I can). I am also trying to find a go-to recipe that I can rely on. Not to mention, I plan to bake the cupcakes for my daughter's 2nd birthday party next month!! I want to experience all of the scratch wasc's goodness!!

Edited: I let the cupcakes sit in the pan for 5 minutes, I then put them in a container and put the lid half way on. I think it may have steamed them because the tops of the cupcakes were sticky. I don't know if that helps but I wanted you all to know exactly what I did.
Thanking you in advance for your time and wealth of knowledge
