For the Chocolate Cupcakes: Preheat oven to 350°F (170°C). Line muffin pan with cupcake liners.
In a large mixing bowl, stir together the granulated sugar, flour, cocoa powder, baking powder, baking soda and salt. Add the butter and mix on low speed until the flour mixture is moistened. Add the eggs one at a time and mix on low speed to combine. Scrape the bowl with a rubber spatula between additions. Blend in the sour cream and vanilla. Beat the batter on medium speed for about one minute.
Fill the cupcake liners a little over half full with batter and bake for 17-19 minutes, until a toothpick inserted into a cupcake comes out clean. Allow the cupcakes to cool for 10 minutes in the pan on a wire rack, then remove them from the pan and cool completely on a wire rack.
For the Peanut Butter Buttercream: In a large mixing bowl, beat peanut butter, butter, vanilla, powdered sugar and salt on medium-low speed with the paddle attachment until creamy. Scrape the bowl. Add the heavy cream and beat on high speed until frosting is light and fluffy. Thickly spread or pipe frosting onto cooled cupcakes.
For the Chocolate Cupcake Toppers: Finely chop the chocolate. Melt two thirds of the chocolate in the microwave or over a double boiler.** Add the rest of the chocolate and stir until smooth. Place melted chocolate into a freezer bag and cut off a tiny piece of one of the corners of the bag. Slide a template under your piece of parchment paper (or wax paper) and pipe the chocolate designs onto the parchment paper. You can spread the chocolate with a toothpick, if necessary. Should your chocolate get too cool or thick to pipe, heat the bag with the chocolate in the microwave for a few seconds, knead it a little and then you can continue using it. Allow the chocolate shapes to harden before releasing them from the paper. Stick the shapes into or lay them over the frosting.
Notes
*Preferably industrially produced peanut butter. Natural, drippy peanut butter is not recommended. **Be sure not to get any water in the chocolate!Chocolate Cupcake recipe slightly adapted fromBaking a Moment. Peanut Butter Frosting recipe slightly adapted fromBarefoot Contessa.