Preheat the oven to 180°C and line two 10 x 8-inch rectangular pans with baking paper.
Sift together flour and baking powder; set aside.
In the bowl of an electric mixer, beat the butter until pale in color. Add the sugar and beat on high speed until creamed together, scraping down the bowl as needed.
Add eggs one at a time, beating well after each addition. Add vanilla and beat to combine.
Reduce mixer speed to medium and add flour mixture alternating with the milk, beat until just combined and smooth. Do not over mix. The batter will become thin.
Divide the batter into two equal portions and add a dollop of pink and green food colorings to each half. Mix well separately.
Pour the prepared batter into the lined trays and bake for about 35-40 minutes, or until a toothpick inserted in the center comes out clean. Remove the cake from the pans and let it cool on a wire rack.
Place the first cake layer at the bottom, and evenly spread a layer of vanilla buttercream. Then, place the second layer on top, and spread another layer of buttercream. Finally, decorate the buttercream frosting.
Trim the edges. If you wish to add more decorations, use the remaining buttercream to pipe as needed.