Make the Cake Layers:
- Prep Oven & Pans: Preheat oven to 350°F (177°C). Butter three 8-inch round cake pans well.
- Mix Dry: In a medium bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.
- Melt Chocolate Mixture: In a saucepan, bring water and sugar to a boil over medium-high heat, stirring until sugar dissolves (~1 min). Remove from heat. Stir in chopped chocolate and butter until smooth and melted. Chill this mixture in the fridge for 10 minutes.
- Combine Wet: Whisk vegetable oil and vanilla into the chilled chocolate mixture. Quickly beat in the eggs until just combined.
- Combine Wet & Dry: Gently whisk the dry ingredients into the wet mixture until just smooth. Important: Do not overmix!
- Bake: Divide batter evenly between the prepared pans. Bake for 20-23 minutes, or until a toothpick inserted into the center comes out clean.
- Cool: Let cakes cool completely in their pans before frosting.
Whip Up the Frosting & Assemble:
- Cream: In a stand mixer (or bowl with a hand mixer), beat the room temperature butter and peanut butter until smooth and creamy.
- Add Flavor: Beat in the vanilla extract and salt.
- Add Sugar: Gradually add the powdered sugar (about ½ cup at a time), beating well on medium speed. Scrape the bowl sides as needed. The mixture will look dry at this stage.
- Get Fluffy: Add the milk, 1 teaspoon at a time, beating on medium speed until the frosting is perfectly light and fluffy.
- Layer Up: Place one cooled cake layer on your serving plate. Spread a thick layer of frosting on top. Repeat with the remaining two layers.
- Crumb Coat: Apply a thin, smooth layer of frosting over the top and sides of the cake (this is the crumb coat). Chill the cake in the freezer for 10 minutes to set.
- Final Frost & Finish: Apply a final, generous layer of frosting smoothly over the cake. Drizzle with chocolate sauce and add any extra decorations if you like!