Sunday Bake-a-thon Part 2- Peanut Butter cupcake

My next cupcake recipe to test was peanut butter.  I wanted to incorporate actual natural peanut butter into the batter.  I do not advocate the use of processed peanut butter - I've never been a fan of skippy or jiff - they just don't cut it.  I had a recipe on had that called for adding chopped peanuts to the batter and I've used this recipe in the past with great success and usually just pipe a peanut butter filling into the cup after its cooled but this time I wanted a peanut butter cake! Prior to adding the flour, I added about 1/2 cup of creamy natural peanut butter to the wet ingredients and mixed until incorporated.  Once I added the dry ingredients (alternating flour mixture with milk in 3 parts each) I noticed the batter was considerable denser than usual so I knew this would need some sort of adjustment to lighten up the cupcake. But before adjusting I baked some off to see the outcome and here was the result:

The top crusted up, resembling very much like a peanut butter cookie.  They were delicious but I wouldn't consider it a cupcake.  And once you bit into it, it kind of stuck to the roof of your mouth like peanut butter does, and being the peanut butter fanatic that I am, I loved loved loved this! 
I adjusted the recipe by thinning it out with about a 1/3 cup of milk and it did lighted up the batter.  Once I baked the second batch, I noticed the top baked up much more smoothly and the cake itself was fluffier (forgot to take photo- sorry).  But the cake was not light enough.  In addition, as I kept these at home over the next couple of days in a tightly sealed container, I noticed they got must harder as opposed to the smore's cupcake which remained as fresh and fluffy as the day I baked them.
I'm going to retest this peanut butter recipe and perhaps add either buttermilk or sour cream instead.  I really want to achieve a moist fluffy cupcake even though it's made with a dense and thick peanut butter.
For the frosting I made an American buttercream using unsalted butter, powdered sugar, vanilla, and cocoa dissolved in boiling coffee/water mixture. The recipes I have found online for American buttercream seem to turn out too loose for piping a pretty swirl so I always increase the ratio of sugar to butter and that seems to do the trick.  I beat for several minutes to eliminate the chance of a grainy texture.  I topped the frosting with crushed peanuts.  I am happy with this test but not fully satisfied - need to strive for perfection.