1. Preheat the oven to 350°. Butter an 11-by-17-inch jelly-roll pan and line the bottom with wax paper. Butter the paper and flour the pan, tapping out the excess flour.
2. In a medium bowl, whisk the flour with the baking powder and salt. In a large bowl, using a handheld electric mixer, cream the butter. Add the sugar and beat until fluffy. Add the eggs, 1 at a time, beating well between additions. Beat in the vanilla and lemon zest. Beat in the dry ingredients in 3 additions, alternating with the milk; beat at medium speed until smooth.
3. Pour the batter into the prepared pan and smooth the surface. Bake for 20 minutes, or until the cake is golden and beginning to brown around the edges. Transfer to a wire rack and let cool.
4. Turn the cake out onto a work surface and peel off the wax paper. Using a 6-inch plate or pot lid as a guide, cut out 3 rounds from the cake. Save the cake scraps for a snack.
5. Beat 1 tablespoon of the apricot preserves into 1/2 cup of the Cream Cheese Frosting. Place one cake round on a plate. Spread it with half the apricot frosting and top with a second cake layer. Spread with the remaining apricot frosting and top with the last cake layer. Brush the remaining melted apricot preserves all over the top and sides of the cake. Refrigerate the cake for at least 10 minutes.
6. Spread the remaining Cream Cheese Frosting all over the top and sides of the cake. Refrigerate until chilled.
7. In a heatproof bowl, mix 3 drops each of red and green food coloring with the water. Sprinkle the gelatin over the mixture and let stand until softened. Place the bowl in a small skillet of boiling water and stir just until the gelatin melts. Using a small paintbrush, paint rounded squares all over the cake to create a giraffe-spot pattern.