In a large bowl, with a mixer at high speed, beat the eggs just until foamy. Add the sugar, then the cooled chocolate mixture and vanilla, and beat just until blended. Stir in the flour mixture just until it disappears, then spoon into the prepared pan. Bake for 35 minutes, or just until set and a toothpick inserted in the center comes out almost clean. Let cool completely in the pan before frosting.