In a medium pot, melt the butter. When the butter has melted, remove the pot from the heat. Add the chocolate and stir until it has melted. Whisk in 3 cups of sugar, 4 eggs and 2 teaspoons of vanilla extract. Stir in the flour until just combined.

In a large bowl, beat the cream cheese until smooth. Add the remaining sugar, beating until combined. Add the final egg and the remaining vanilla extract; mix until well blended.

Pour half of the chocolate mixture into your prepared pan; smooth with a spatula. Top with the cream cheese mixture. Pour the remaining half of the chocolate mixture on top of the cream cheese mixture.

Place in the oven. Bake for 30 minutes, or until a toothpick poked in the center comes out clean. Remove from oven and allow to cool completely before cutting.