Preheat an oven to 350 degrees F (175 degrees C). Line a 9×13-inch baking dish with parchment paper.
Sift together the flour and baking powder in a bowl; set aside. Beat 1 cup plus 2 tablespoons margarine with an electric mixer in a large bowl until fluffy. Beat in the white sugar, vanilla extract, and 5 egg yolks. Stir in the flour mixture until just combined.
Beat egg whites until foamy in a large glass or metal mixing bowl. Continue to beat until stiff peaks form. Lift your beater or whisk straight up: the egg whites should form a sharp peak that holds its shape. Use a rubber spatula or wire whisk to gently fold 1/3 of the egg whites into the batter, keeping as much volume as possible. Add the remaining egg whites, folding just until incorporated.
Pour half of the batter into a separate bowl. Sift the cocoa powder into one bowl of batter. Stir until just combined. Pour the white batter into the prepared baking dish, then spread the chocolate batter over. Arrange the tart cherries on top.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 25 to 30 minutes. Cool in the pans for 10 minutes before removing to cool completely on a wire rack.
To make the buttercream, beat 1 cup plus 2 tablespoons margarine, confectioners’ sugar, rum, and 2 egg yolks until light and fluffy. Spread over cooled cake. Refrigerate cake until ready to serve.