The cake has a tart flavor from the orange zest and is filled with sweet orange marmalade. You may replace the orange with lemon zest and curd, or omit the zest and fill the cake with other types of jam.
Preheat oven to 350 degrees. Line an 11-by-17-inch rimmed baking sheet with parchment paper; butter parchment and sides of pan, and dust with flour. Set aside.
In a small bowl, whisk together milk, eggs, vanilla, and orange zest. In the bowl of an electric mixer fitted with the paddle attachment, combine flour, sugar, baking powder, and salt. With mixer on low, add butter 1/2 stick at a time; beat until mixture is the texture of coarse meal. Add half the milk mixture, and increase speed to medium-high; beat 1 minute. With mixer running, slowly add remaining milk mixture. Beat 30 seconds more.
Spread batter into prepared pan, smoothing top with a spatula. Bake until golden on the top and edges are pulling away from the sides, 20 to 25 minutes.
Transfer to a wire rack; let cool 10 minutes. Run a knife around the edge of the pan, and invert onto wire rack. Remove parchment; let cake cool completely.
Cut cake in half, into two 8 1/2-by-11-inch pieces. In a small saucepan, heat marmalade until it is syrupy (strain as needed). Spread over one half; let cool at least 10 minutes. Top with remaining half.
Cut cake into 8 pieces, each about 3 by 4 inches. Cut parchment paper into 8 pieces, each 4 by 5 inches; set a piece of cake on each. Measure 2 1/2 cups buttercream; set aside. Using a small offset spatula, thinly coat top and sides of each piece with some of the remaining buttercream to seal in crumbs. Transfer cakes on parchment paper to a baking sheet, and place in refrigerator 20 to 30 minutes. Remove from refrigerator; cover each piece with a final layer of buttercream, smoothing top and sides with the offset spatula. Return baking sheet to refrigerator.
Place 3/4 cup reserved buttercream in a small bowl; tint it red for the numbers. Transfer to a pastry bag fitted with an Ateco #11 large round tip. Divide remaining 1 3/4 cups reserved buttercream among four small bowls; tint each a different color. Transfer all but one color to separate pastry bags fitted with #11 tips. Place remaining color in a pastry bag fitted with an Ateco #5 small round tip to make tiny dots. Remove cakes from refrigerator. Using red frosting, pipe numbers 1 through 8 on top of cakes, a different number on each. Pipe dots around numbers with other colors. Return cakes to refrigerator.
To assemble, remove cakes from refrigerator, and remove parchment paper. Arrange cakes on a serving tray, forming a hopscotch or other desired pattern. Serve.
To decorate the cake pieces, we used red, orange, yellow, turquoise, and royal-blue gel-paste food colors.