Reese's Peanut Butter Cake

From: Annie's Eats

I made this cake for a friend's birthday this past weekend, and it was a big hit with everyone. I had to take lots of pictures of it, because it was the most impressive-looking cake I think I've ever made.
For the cake:

2 cups all-purpose flour
½ cup unsweetened cocoa powder
½ tsp. baking powder
½ tsp. baking soda
½ tsp. salt
2 sticks unsalted butter, at room temperature
1 ½ cups sugar
2 large eggs
2 large egg yolks
1 tsp. vanilla extract
1 cup buttermilk
4 oz. bittersweet chocolate, melted and cooled (optional)

For decorating:
1 ½ - 2 batches peanut butter frosting (below) (I made a double batch and had about 1.5 cups leftover)
miniature Reese’s cups, halved and/or chopped

For the cake, center a rack in the oven and preheat the oven to 350°. Butter two 9×2” round cake pans, dust the insides with flour (I used cocoa powder), tap out the excess and line the bottoms with rounds of parchment paper (don’t skip the parchment paper. I only used parchment in one pan, and that layer came out of the pan ok. The one without parchment stuck to the bottom and I end up with an uneven cake as a result). Place the pans on a baking sheet.

In a medium bowl, whisk together the flour, cocoa powder, baking powder, baking soda and salt. Set aside.

In a stand mixer fitted with the paddle attachment, beat the butter on medium speed until soft and creamy. Add the sugar and beat for about 2 minutes, until thoroughly blended into the butter. Add the eggs and yolks one at a time, beating for one minute after each addition and scraping down the sides of the bowl as needed. Beat in the vanilla. Reduce the mixer speed to low and add the dry ingredients alternately with the buttermilk; add the dry ingredients in 3 additions and the buttermilk in 2 (beginning and ending with the dry ingredients). Mix each addition only until it is blended into the batter. Scrape down the bowl and add the melted chocolate, if using, folding it in with a spatula. Divide the batter between the prepared cake pans.

Bake for 26-30 minutes or until the cakes feel springy to the touch and start to pull away from the sides of the pans (One of my layers needed about 32 minutes, and the other needed closer to 40 minutes, but definitely check periodically because all ovens have their own quirks). Transfer to wire racks to cool for about 5 minutes, then run a knife around the sides of the cakes, unmold them and peel off the paper liners. Invert and cool to room temperature right side up.

To assemble the cake, place one cake layer on the serving dish, or cover a cardboard circle with foil to use as a serving tray. Spread peanut butter frosting on top of the cake layer. (If desired, sprinkle with chopped Reese’s cups.) Place the second cake layer on top of the frosting. Frost the top and outside of the cake with remaining peanut butter frosting. Decorate with halved and chopped Reese’s cups as desired.

Peanut Butter Frosting
From Ina Garten

1 cup confectioners' sugar
1 cup creamy peanut butter
5 tablespoons unsalted butter, at room temperature
3/4 teaspoon pure vanilla extract
1/4 teaspoon kosher salt
1/3 cup heavy cream

Place the confectioners' sugar, peanut butter, butter, vanilla, and salt in the bowl of an electric mixer fitted with a paddle attachment. Mix on medium-low speed until creamy, scraping down the bowl with a rubber spatula as you work. Add the cream and beat on high speed until the mixture is light and smooth.