To make the cake, preheat the oven to 350˚ F. Butter the edges of 3 8-inch round cake pans and dust with cocoa powder, shaking out the excess. Line the bottoms of the pans with parchment paper. In a medium bowl, combine the cocoa powder, espresso powder, and boiling water. Whisk until smooth; set aside to cool slightly. When cooled down a bit, whisk in the sour cream and vanilla. Set aside. In the bowl of an electric mixer, beat the butter on medium-high speed until smooth, about 1 minute. Gradually blend in the sugar and whip on medium-high speed until light and fluffy, 2-3 minutes more. Blend in the eggs one at a time. In a medium bowl, combine the flour, baking soda, and salt; whisk to blend.

With the mixer on low speed, add the dry ingredients in three additions alternating with the sour cream mixture, beating each addition just until incorporated. Divide the batter evenly between the prepared pans. Bake the cake layers for about 30-32 minutes or until a toothpick inserted in the center comes out clean, rotating the pans during halfway through to ensure even baking. Transfer the baked cake layers to a wire rack and let cool in the pans at least 30 minutes before inverting onto the rack to cool completely.

To make the frosting, combine the heavy cream and ¼ cup of the confectioners’ sugar in the clean bowl of an electric mixer fitted with the whisk attachment. Whip on medium-high speed until light, fluffy, and stiff peaks form, being careful not to over mix. Transfer the whipped cream to a separate bowl. In the now empty mixer bowl fitted with the paddle attachment, combine the butter and peanut butter. Beat on medium-high speed until smooth, about 45 seconds. Add the remaining confectioners’ sugar to the bowl and mix in, slowly at first until incorporated, then increasing the mixer speed to high. Blend in the vanilla extract and salt, and continue to whip on high speed until very fluffy, about 4-5 minutes, scraping down the sides of the bowl as needed. Use a large spatula to gently fold about a third of the whipped cream into the peanut butter frosting. Once the first addition has been evenly incorporated, gently fold in the remaining whipped cream until no streaks remain.