Preheat oven to 350° F. Generously butter two 8-inch cake pans. Dust with cocoa powder; turn and tap pans lightly over the sink to release excess cocoa.
Place chocolate in a double boiler and melt over medium-low heat, stirring occasionally, until melted. Remove from heat and mix in cocoa powder 1 tablespoon at a time. Stir until smooth and set aside.
Whisk flour, baking soda, and salt together in a bowl and set aside.
Pour raspberry liqueur/wine into a 2-cup glass measure with a pour spout. Spoon in sour cream until the the level of both ingredients reaches the 2-cup line. Mix well and set aside.
In the bowl of a standing electric mixer, cream together butter and sugar until light and fluffy.
Add the eggs to the butter mixture one at a time, beating after each addition. Beat until well blended. Add the vanilla and chocolate mixture and stir until well combined.
Add the flour mixture, alternating with the sour cream mixture, to the egg-butter mixture. Stir until the dry ingredients are just moist and blended, taking care not to over mix.
Divide the thick batter between prepared pans, smoothing the top to even out the batter. Bake until the cake is firm to the touch and a skewer inserted into the center comes out with just a few crumbs clinging to it, about 35 to 40 minutes.
Remove from the oven and cool the cakes for 15 to 20 minutes in their pans. Carefully turn the cakes out onto a rack and cool completely.
For the buttercream, place raspberries in a mesh strainer over a bowl. Press the berries against the side with a spatula to release their juices into the bowl.
In the bowl of a standing electric mixer, combine butter, 3 cups of the confectioners' sugar and the raspberry juice. Mix slowly with the paddle attachment to combine. Add another cup of confectioners' sugar and mix until incorporated. Increase speed to medium-high and beat until smooth. Add additional powdered sugar about ½ cup at a time to achieve a smooth, creamy consistency that will hold up on the cake. Mix in milk or cream to thin frosting if needed.
To assemble, trim mounded tops of cooled cakes. Place one layer on a cake stand or serving plate. Using a a 1/2-inch round tip, pipe dollops of buttercream around the edge of the cake to make a border. Spoon jam within the border and spread it evenly. Chill cake for about 20 minutes before proceeding. Pipe a thin layer of buttercream over the jam and top with remaining cake layer. Working from the outside in, pipe dollops of buttercream over the top. Garnish with fresh raspberries.