Birthday Cupcake Attempt

Baking is tough and time-consuming.  Even though I try and follow directions exactly and measure everything out carefully, my baked desserts never come out as I would want.  I partially blame my oven, but I think I must be responsible as well.  K.'s birthday is coming up so I wanted to bake his favorite cupcakes for him.  K.'s ideal cupcake is yellow cake with chocolate frosting, which I've never made before, so I scoured the internet before coming across the Vanilla Cupcake recipe, from Billy's Bakery of NYC, which was the best recipe according to thekitchn, which I generally trust.  For the cupcake part of it, I pretty much followed the recipe exactly, since this was something I've never made before.  It was just enough batter for two trays of cupcakes for a total of 24 cupcakes, even though the recipe said it would make 30 cupcakes.  I had to bake it for close to 30 minutes, as opposed to 17-20 minutes according to the recipe.  It could just be my oven though; it always bakes unevenly, which is an issue for desserts. I had more difficulty finding a good chocolate buttercream frosting recipe however, so I took a chance and combined a couple of recipes starting with the Billy's Cupcakes recipe, for my own spin on it.  I thought the cupcakes were a little dense. I may have overbaked them a bit; I expected them to be fluffier.  Aside from the oven, my baking powder may have been too old.  I liked the frosting though; it was nice and airy with just enough chocolatyness and not too much sugar (the original recipe called for 6-8 cups of confectioners sugar, almost 2x what I used)

Vanilla Cupcakes (from Billy's Cupcakes via 52 cupcakes):

• Over low speed with the paddle attachment in my Kitchenaid stand mixer, mix together 1 3/4 cups of cake flour (I used King Arthur cake flour from Whole Foods), 1 1/4 cups of unbleached all-purpose flour, 2 cups of granulated sugar, 1 TB of baking powder and 3/4 tsp of salt (I also sifted all these items together)
• Add two sticks of room temperature butter and mix until there really aren't any clumps of butter left
• In a medium bowl, whisk together 4 large eggs, 1 cup of whole milk, 1 tsp of vanilla extract (wet ingredients)
• Turn the stand mixer to medium and add in the wet ingredients, about 1/3 or less at a time.  Scrape the bowl if necessary to make sure everything is combined
• Using an ice cream scoop, scoop the batter into cupcake tins with paper liners, about 2/3 of the way filled
• Bake at 325 degrees, rotating at least once.  I think 25 minutes should be enough, but start checking with a toothpick at 20 minutes. As soon as the toothpick comes out clean, cupcakes are done. Remove cupcakes carefully from the tin and let cool completely on a rack

Chocolate Buttercream Frosting Recipe (enough for 24 cupcakes with about 1 cup leftover)

• Over low speed with the paddle attachment in my Kitchenaid stand mixer, whip 2 sticks of room temperature butter for about 2-3 minutes
• Over medium speed, add 4 cups of confectioners (powdered) sugar, 1 tsp of vanilla extract, 1 TB of espresso, 1/2 cup of milk, 3/4 cup of unsweetened cocoa powder and mix all together until smooth