1. Preheat oven to 350 degrees. Butter two 9-by-2-inch round pans; line with parchment paper. Butter parchment, and dust with cocoa. Sift together the flour, cocoa, baking powder, baking soda, and salt in a medium bowl, and set aside.

2. Place butter in bowl of electric mixer fitted with the paddle attachment. Beat on medium speed until creamy. Gradually add the sugar, and beat until fluffy. Add the eggs one at time, incorporating well after each addition; add the vanilla.

3. Add the flour mixture and sour cream alternately, starting and ending with the flour mixture. Scrape down sides of bowl as needed. Divide the batter between pans; bake until a cake tester inserted in the center comes out clean, about 30 minutes.

4. Cool cakes in pans on wire racks for 10 minutes. Unmold, and cool completely, right side up.

5. Heat marmalade just until it is a spreadable consistency. Choose the more attractive cake layer, and set aside. Place the other layer on an 8-inch cake round, bottom side up. Using a pastry brush, coat the cake with the kumquat poaching syrup. Spread the marmalade over the cake, and allow to set, about 10 minutes. Brush the bottom of the reserved layer with the kumquat poaching liquid, and place on top of the marmalade, syrup side down. Dust the top with cocoa, and garnish with candied kumquats and sugared bay leaves.