Preheat oven to 400 degrees. Lightly oil a 10-inch round cake pan, line bottom with parchment paper, spray oil on the parchment and dust with flour.
Over low heat, melt butter with chocolate. Remove from heat. Combine sugar with cocoa and whisk into chocolate. Whisk in sour cream, extract and egg yolks.
In a separate bowl, whip egg whites to soft peaks and fold into chocolate one third at a time. Turn mixture into cake pan, set in a large roasting pan, and pour in enough hot water to go 1 inch up sides of cake pan. Bake for 10 minutes, then reduce temperature to 325 degrees and bake for an additional 40 to 50 minutes; the center will appear soft and the sides firm. Remove from oven and cool on a rack. Refrigerate cake overnight.
The next day, remove cake from pan. Invert a plate on top, then flip pan over. Peel off parchment paper. Spread raspberry jam (mixed with the liqueur, if using) over top, then smother with raspberries. Serve with dollops of whipped cream.
NOTE: To make clean slices, use a knife warmed in hot water, then dried with a towel; clean knife and repeat for each slice.