Donauwelle is a German and Austrian sheet cake. It consists of two pound cakes, one on the bottom, flavored with vanilla, and one on top of it, flavored with chocolate. Cherries, buttercream, and chocolate ganache are then added to the top, and as the cake bakes, the tart cherries will fall down through the batter, making a wavy pattern (maybe reminiscent of the Danube river?) in the cross-section of the cake.