Preheat oven to 325 degrees with a rack in the center. Butter a 13-by-9-by-2-inch cake pan. Line bottom and long sides of cake with parchment paper, allowing at least a 1-inch overhang; set aside. In a medium bowl, sift together flour, baking powder, salt, and baking soda; set aside.
In the bowl of an electric mixer fitted with the paddle attachment, beat butter and sugar on medium speed until light and fluffy, 3 to 5 minutes. With the mixer running, add eggs, one at a time, beating to incorporate after each addition. Beat in vanilla, and lemon juice and zest.
With mixer on low speed, add flour mixture, alternating with buttermilk, beginning and ending with the flour mixture, and beating until fully incorporated.
Pour into prepared cake pan. Bake until light golden brown and a cake tester inserted in the center comes out clean, 25 to 30 minutes. Transfer to a wire rack to cool.
Place 3 cups buttercream in a medium bowl, and 1/2 cup in each of 2 bowls. Add sunset orange and blushing pink food coloring to the medium bowl, to make peach; set aside. Add golden yellow to a bowl with 1/2 cup buttercream; place in a pastry bag fitted with a #18 Ateco tip and set aside. Tint remaining 1/2 cup buttercream bright yellow, and place in a pastry bag fitted with a #6 Ateco tip and set aside.
To assemble: Cut out 6 four-inch rounds from cake. Split each round horizontally, creating twelve layers. Spread top of one layer with 3 tablespoons of the peach buttercream. Place a second layer on top of buttercream, pressing gently to adhere. Repeat with remaining layers, creating six filled cakes.
To make the petals, place a 3 1/2-inch round cutter 1/2 inch in from the edge of one of the cakes, and cut; discard the cut piece. Repeat with four more cakes. Reserve one uncut cake for center of flower.
Using an offset spatula, frost each of the cakes with 1/2 cup peach buttercream. Start from the center, and work out, making sure to push the buttercream over the sides of the cake. Smooth the buttercream.
Place the center of the flower on a serving plate or pedestal. Coat the sides of remaining cakes with sprinkles, and place evenly around the pedestal, being sure to position the cut section of each cake against the center cake to adhere.
Using the piping bag filled with golden buttercream, pipe two rows of small stars around the perimeter of the center of the flower. Using the piping bag filled with yellow buttercream, pipe small dots onto the center of the cake until it is completely covered.