Preheat oven to 325 F. Either butter or line with parchment paper, (2) 5-inch mini springform pans; set aside.
In a bowl, melt butter, in microwave. In a small bowl, sift flour, baking powder & salt; add to butter & combine.
In a small bowl, whisk together egg yolks, milk & vanilla then add to butter mixture. Whisk until combined.
Beat egg whites until foamy then gradually add sugar; beat until stiff peaks form. Fold into batter.
In a small cup with a spout, place about 1/2 cup of the batter & add cocoa powder. Fold in to combine.
Divide white batter between 2 prepared pans. Pour chocolate batter onto each cake forming circles with it. Using a wooden skewer, make lines from the center out making a spider web design.
Bake for 45 minutes on bottom rack of the oven or until cake tests done with a wooden pick. Remove from oven, allow to cool for about 5 minutes on cooling rack. Flip over & cool in pans for at least 30 minutes. We enjoyed a small bit of raspberry puree with the cake.