Preheat the oven to 350°F (175°C) and line a 12-cup muffin tin with cupcake liners.
In a microwave-safe bowl, melt the chocolate by heating it at 50% power in 20-second intervals. Stir between each interval until the chocolate is smooth and fully melted, then set aside to cool slightly.
In a mixing bowl, whisk together the flour, cocoa powder, baking soda, and salt for about 20 seconds, then set aside.
Using an electric hand mixer, combine the sugar and butter in a large mixing bowl. Mix until the ingredients are well incorporated, noting that it may not come together completely. Add the egg, egg yolk, and vanilla extract, then mix in the melted chocolate and sour cream until smooth.
Gradually incorporate half of the flour mixture, mixing at a low speed until combined. Add the remaining flour mixture and mix until just incorporated. Slowly add the boiling water, about 1/5 at a time, mixing well after each addition. Continue mixing on medium-low speed for around 1 minute until the batter is smooth.
Divide the batter evenly among the prepared muffin cups, filling each about 2/3 full (approximately 1/4 cup of batter). Bake in the preheated oven for 18 to 20 minutes, or until the cupcakes are set and a toothpick inserted into the center comes out clean.
Once baked, remove the cupcakes from the oven and transfer them to a wire rack to cool for about 10 minutes. After cooling, place them in an airtight container (making sure the tops do not touch) to seal in moisture, then allow them to cool completely before frosting with raspberry buttercream.
Top with fresh raspberries before serving. Store any leftovers in the refrigerator in an airtight container.