¾cup12 Tablespoons unsalted butter, cut into tablespoons, at room temperature
MAKE THE CUPCAKES: Move the oven rack to the middle position and preheat oven to 350 degrees F. Line a muffin tin with 12 paper liners and set aside.
In a small heatproof bowl add the butter, chocolate, and cocoa powder. Place the bowl over a saucepan with two inches of simmering water. Slowly stir the butter and chocolate until melted and combined, and then set the bowl aside until the chocolate is warm to the touch.
In a small bowl whisk together the flour, baking powder, and baking soda until combined.
In a large bowl whisk together the eggs, sugar, vanilla, and salt until fully incorporated. Next, add the cooled chocolate to the egg mixture and whisk until combined. Then, sift in one-third of flour mixture and whisk until combined; whisk in sour cream until combined. Finally, sift in the remaining flour mixture and whisk until the batter is combined and no flour streaks remain.
Fill the cupcake liners 2/3 full with batter. Bake until a toothpick inserted into the center of a cupcake comes out clean, about 18 to 20 minutes.
Let the cupcakes cool for 10 minutes, and then remove them from the muffin tin to cool on a wire rack until completely cool, about 20-30 more minutes.
MAKE THE STRAWBERRY SWISS MERINGUE BUTTERCREAM: Puree the strawberries in a food processor; set aside.
In the bowl of a stand mixer set over a small pot of simmering water, whisk together the egg whites, sugar, and salt. Continue whisking until the sugar dissolves (the mixture should feel completely smooth when rubbed between your fingertips).
Next, attach the bowl to the stand mixer and fit it with the whisk attachment. Starting on low speed and gradually increasing to medium-high speed, whisk the mixture until stiff (but not dry) peaks form. Continue mixing until the mixture is light, fluffy, glossy, and completely cool (test by touching the bottom of the mixing bowl), this will take about 10 minutes.
Then, with the mixer running on medium-low speed, add the butter 2 tablespoons at a time, mixing well between each addition. Scrape down the sides of the bowl, increase the speed to medium, and mix until the buttercream comes together (it can take up to several minutes, but it will come together eventually). Next, switch to the paddle attachment and beat at medium-low speed to remove air bubbles, about 2 minutes. Gradually add the strawberry puree, and mix to combine.
TO FINISH: Finally, frost the cupcakes as desired, and garnish with strawberry slices just before serving.