Preheat oven to 325°F. Spray 2 (9-inch) round cake pans with baking spray with flour.

In the bowl of a stand mixer fitted with the paddle attachment, beat butter, sugar, and vanilla at medium speed until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Pour in melted chocolate. Beat in egg yolks.

In a medium bowl, beat egg whites with a mixer at medium-high speed until soft peaks form. Fold egg whites into chocolate mixture alternately with flour until combined. Pour batter into prepared pans.

Bake until a wooden pick inserted in center comes out clean, 15 to 20 minutes. Let cool in pans for 10 minutes. Remove from pans, and let cool completely on wire racks.