Heat oven to 350℉/177℃.
Spray 8 six-ounce ramekins with baking spray. Set aside.
Combine chocolate pieces and butter in a microwave-safe bowl.
Microwave in 30 second intervals, stirring after each, until both chocolate and butter are melted and smooth. Set aside.
In a large bowl, whisk together eggs, sugar, and vanilla.
Beat in cooled chocolate mixture.
Gently stir in flour.
Fill each ramekin half full.
Place a frozen plug of ganache on each and press down slightly.
Cover with remaining batter.
Place on a sheet tray and place in the oven.
Bake for 12-15 minutes, until cake is just starting to pull away from the edges of the ramekin.
Remove from oven.
Invert gently onto plate.