Step 1 – Preheat oven to 180C. Grease a 26 x 16cm slice pan with butter. Line with baking paper, allowing the sides to overhang.
Step 2 – Use an electric beater to beat the sugar, butter and vanilla in a large bowl until pale and creamy. Add eggs, 1 at a time, beating after each addition until just combined. Sift in the combined flour. Add the milk. Fold until well combined. Divide evenly among 3 bowls. Add enough food colouring to 1 bowl to make a dark pink batter. Repeat with the remaining batter, adding less food colouring as you go, to make pale pink and light pink batters.
Step 3 – Spoon the dark pink batter into the lined pan. Use a palette knife to spread as evenly as possible. Bake for 12-15 minutes or until a skewer inserted into the centre comes out clean. Set aside for 5 minutes in the pan to cool slightly before transferring to a wire rack to cool completely. Repeat, in 2 batches, with the light pink and pale pink batters, relining the pan with baking paper between batches.
Step 4 – Trim the tops of the cakes, if necessary. Spread the dark pink cake with 1/2 cup of frosting. Top with the pale pink cake. Spread with 1/2 cup of frosting and top with the light pink cake. Use a large serrated knife to trim the sides. Cut into 8 squares. Spread the sides and top of each cake with the remaining frosting. Decorate with flowers, cachous and letters.