Make the cupcakes
Position a rack in the center of the oven and heat the oven to 350°F. Line 20 standard-size muffin cups with paper liners.
Whisk together the flour, baking soda, and salt in a medium bowl.
In another medium bowl, whisk the cocoa powder, water, and molasses until smooth. Let the mixture cool until just warm, and then whisk in the sour cream and vanilla.
In a stand mixer fitted with the whisk attachment (or in a large bowl using a hand-held electric mixer), beat the sugar and butter on medium-high speed until fluffy, about 3 minutes. Reduce the speed to low and add the eggs, one at a time, scraping the bowl after each addition.
Alternately mix in the flour and cocoa mixtures, beginning and ending with the flour in three additions. By hand, stir in the chopped chocolate.
Divide the batter among the cupcake liners (about 1/4 cup each). Bake until the tops spring back when touched and a toothpick inserted in the center comes out with moist crumbs attached, 18 to 20 minutes. Cool in the pan for 5 minutes, and then remove from the pan and cool completely on a wire rack.
Make the buttercream
Put the egg whites in the bowl of a stand mixer fitted with the whisk attachment (or put them in a large bowl and have ready a handheld electric mixer).
In a 1-quart, heavy-duty saucepan, stir the sugar and 1/2 cup water. Cook over medium-high heat, stirring, until the sugar is dissolved and the mixture begins to boil. Cover the pan and boil for 1 minute to allow the condensation to wash any sugar crystals off the sides of the pan. Remove the lid and place a candy thermometer in the syrup.
When the syrup reaches 215°F, begin beating the whites on medium-high speed until soft peaks form, about 1 minute. At this point, the sugar syrup should reach 234°F. If so, with the mixer running, carefully pour the boiling sugar syrup down the side of the bowl. (If not, turn the mixer off until the syrup reaches temperature and turn it on before adding the syrup.) With the mixer running, add the vanilla, molasses, and salt. Continue beating until glossy, stiff peaks form and the mixture cools to just warm, about 5 minutes.
Reduce the speed to medium low. Beat the butter into the meringue a few pieces at a time (you don’t have to wait until fully incorporated before adding more), until all the butter is incorporated; don’t worry if the buttercream looks curdled at this point. Add the peanut butter and beat until the mixture is smooth and fluffy.
Frost the cupcakes with the buttercream, and garnish with the peanuts and chocolate, if using.
Make Ahead Tips
You can bake the cupcakes a day before frosting them; keep covered at room temperature.
The frosted, garnished cupcakes can be stored, covered, at cool room temperature, for up to 2 days.
nutrition information (per serving):
sat fat g
Photo: Scott Phillips