Make the Chocolate Truffle Center. Place 8 ounces chopped semisweet chocolate in a small bowl. Bring 3/4 cup heavy cream to a boil in a small saucepan. Pour over chopped chocolate. Let sit 5 minutes and then whisk until smooth.
Pour mixture onto a baking sheet lined with wax paper and use a rubber spatula to spread into a smooth layer, almost to the edges of pan. Place in refrigerator for 30 minutes or freezer for 15 minutes.
Line a plate with wax paper. Remove chocolate/cream mixture from refrigerator and portion into 12 heaping tablespoon size portions. Wearing disposable gloves, roll each into a ball and place on plate. Place in freezer while preparing cake batter.
Make Cake Batter. Preheat oven to 325 degrees. Coat 12 individual nonstick muffin cups with melted butter.
Sift together flour and cocoa powder onto a large sheet of wax paper.
Melt 8 ounces chopped semisweet chocolate and the butter in a double boiler and stir until smooth. (I just use a heavy bottomed saucepan set over fairly low heat. You could also use the microwave.)
Place 3 eggs, 2 egg yolks, and 1/2 cup sugar in a bowl of an electric mixer fitted with a paddle attachment. Beat on medium-high speed for 2 minutes.
With mixer on low, gradually add melted chocolate/butter mixture. Gradually add flour mixture.
Once incorporated, stop mixer and scrape down sides. Add vanilla and mix on medium to combine. Use a rubber spatula to make sure batter is evenly mixed.
Pour 3 heaping tablespoons of batter into each muffin cup. Place in center of oven for 5 minutes.
Remove truffles from freezer. Remove muffin tray from oven and place a truffle in the center of each cake, pressing halfway down into batter. Return to oven and bake for 16 to 18 minutes.
Let cool at room temperature for 20 minutes. Give cakes a slight twist to loosen and they should easily come out of the cups. Serve while still warm or rewarm in the microwave.