Prep pans and oven: Preheat the oven to 350°F and grease two 8-inch round cake pans, lining the bottoms with parchment paper for easy release.
Cream butter and sugar: Beat 1 cup softened butter with the granulated sugar for about 3 minutes, until the mixture turns pale and fluffy and no longer looks grainy.
Add eggs and vanilla: Beat in the eggs one at a time, mixing just until each is incorporated, then stir in 1 tablespoon vanilla extract.
Mix dry ingredients: In a separate bowl, whisk together the flour, baking powder, and salt until evenly combined.
Alternate dry and wet: Add the flour mixture to the butter mixture in three additions, alternating with the milk, beginning and ending with flour, mixing on low until just combined.
Divide and tint batter: Split the batter evenly among four bowls and tint each a soft pastel shade with gel food coloring, stirring gently to avoid overmixing.
Layer batter in pans: Spoon dollops of the different colors into the two prepared pans in a random pattern, dividing the batter evenly between the pans.
Bake the layers: Bake for 24 to 28 minutes, until a toothpick inserted in the center comes out clean and the tops spring back when lightly pressed.
Cool completely: Let the cakes cool in the pans for 10 minutes, then turn out onto a wire rack and cool fully before frosting, about 1 hour.
Make the buttercream: Beat the 1 1/2 cups softened butter until smooth, then gradually add the powdered sugar, heavy cream, and remaining vanilla extract, beating until light and spreadable.
Stack and crumb coat: Place one cake layer on a serving plate, spread a thin layer of buttercream on top, stack the second layer, then coat the whole cake in a thin layer of frosting and chill for 15 minutes.
Tint the piping frosting: Divide the remaining buttercream into small bowls and tint each a pastel shade to match your batter colors.
Pipe the flowers: Using a petal tip, pipe overlapping half-moon strokes in a circle to form roses, then fill gaps with small dabs of frosting using a star tip for leaves.
Chill before slicing: Refrigerate the finished cake for 20 minutes to set the piping, then slice with a warm knife for clean edges.