1. To make the cupcakes: preheat the oven to 350 degrees F. Line a 12-cup muffin tin with paper liners. Set aside.
2. Place butter, chocolate and cocoa powder in a medium heatproof bowl. Microwave in 30 second spurts on 50% power, stirring in between, until the butter and chocolate are melted and the mixture is smooth. (Alternatively, you could do this step in a double boiler.) Set the mixture aside to cool until it is warm to the touch.
3. In a small bowl, whisk together the flour, baking soda, and baking powder. Whisk the eggs in a medium bowl to combine, then add the sugar, vanilla and salt and continue whisking until fully incorporated. Add the cooled chocolate mixture to the bowl and whisk to combine. Sift one-third of the flour mixture into the bowl and whisk gently to combine. Add the Greek yogurt or sour cream and whisk to combine. Finally, sift the the rest of the flour mixture into the bowl and whisk until everything is well combined. The cupcake batter will be thick.
4. Divide the batter evenly among the muffin tins, filling each about 2/3 full. Bake until a toothpick inserted into the center of the cupcakes comes out clean, about 18-20 minutes. Remove the pan to a wire rack and let the cupcakes cool for a few minutes before removing them to the rack to cool completely.
5. While the cupcakes are cooling, make the raspberry buttercream frosting. In a food processor or blender, puree the raspberries. Pour raspberries through a fine sieve into a bowl, pressing on solids, to eliminate the seeds. Place butter, half of the powdered sugar, and raspberry puree into mixing bowl. Beat over low speed until well-blended. Add the other half of the sugar, increase speed to medium. Mix until light and fluffy.
6. Frost cooled cupcakes with the raspberry buttercream and top each cupcake with a fresh raspberry, if desired.