To make the cake:

1. Whisk cocoa powder and hot water together in a bowl and set aside to cool. In another bowl, whisk together the dry ingredients (flour, baking powder, baking soda, salt).
2. Mix room temperature butter and sugar together at medium speed until creamy, about two minutes. Add eggs one by one, beating on low speed for about 30 seconds after each addition. Add vanilla and mix to combine.
3. Add a third of the dry ingredients to the mixing bowl, mixing on the lowest speed until you don't see any flour in the batter.
4. Add half of the cocoa powder, mixing on the lowest speed to combine. Repeat with another third of the dry ingredients, then the remaining cocoa powder mixture, then the remaining dry ingredients.
5. Grease three 6" pans and divide the cake batter between them. Bake at 175ºC or 350ºF for 22 minutes or until a toothpick inserted into the middle comes out clean.
6. Leave cake layers in the pans for 10 minutes before turning out onto a wire rack to cool.

To make the peanut butter cups:

1. In a small bowl, melt chocolate in the microwave at 70% power for 30 seconds. Stir and microwave at 70% power for another 30 seconds. Stir until smooth.
2. Line a mini cupcake pan with 8-12 mini cupcake wrappers (depending on how many swirls you want to pipe onto your cake).
3. Spoon melted chocolate into each mini cupcake wrapper to cover the bottom. Use two spoons to add the  peanut butter mixture on top and then cover with the remaining melted chocolate.
4. Refrigerate for at least 30 minutes to set the chocolate and then peel the wrappers off the peanut butter cups and refrigerate.

To make the buttercream filling and frosting:

1. In a mixer with a beater (paddle) attachment, mix room temperature butter with a quarter of the powdered sugar and salt on the lowest speed until incorporated, about one minute.
2. Scrape down to the bottom of the mixing bowl with a spatula to loosen any butter and sugar and add the next quarter of powdered sugar and mix for another minute on low. Repeat with the next quarter of sugar and then the final quarter.
3. Add the vanilla once everything is mixed together and continuing to mix until it's incorporated.
4. Add the milk or cream and mix for about 30 seconds until it's combined with the buttercream. Check the consistency and add more milk or cream a tablespoon at a time until the buttercream is smooth and easily stirred.

To put it all together:

1. Attach the first layer of cake to a cake board with a dot of buttercream. Spread 1/3 cup of chocolate buttercream on top of that layer and then place another cake layer on top. Spread another 1/3 cup of chocolate buttercream as filling and place the final cake layer on top.
2. Cover the cake with a thin layer of peanut butter buttercream (a "crumb coat") and then let this set, which will take one hour at room temperature or 30 minutes the fridge or 15 minutes in the freezer.
3. Frost the cake with peanut butter buttercream, smoothing the frosting with a cake comb.
4. In a piping bag fitted with a star shaped piping tip, spread the remaining 1/3 cup of chocolate buttercream in a line from the bottom of the bag to the top. Fill the rest of the bag with peanut butter buttercream. Pipe swirls of the two-tone buttercream onto the top of the cake and push a peanut butter cup into each one.
5. Refrigerate for up to two days. Take the cake out of the fridge at least 2 hours before serving so that it has time to come to room temperature, when it tastes best.