01 - Preheat oven to 350°F. Grease an 8x8-inch baking pan and line with parchment paper.
02 - Whisk together flour, baking powder, baking soda, and salt in a medium bowl until well blended.
03 - Beat softened butter and granulated sugar in a large bowl until light and fluffy, approximately 3-4 minutes.
04 - Add eggs one at a time, beating thoroughly after each addition. Mix in vanilla extract.
05 - Add one-third of dry ingredients to butter mixture, mixing gently. Add half the buttermilk, then another third of dry ingredients. Add remaining buttermilk, then final third of dry ingredients. Mix just until combined.
06 - Fold crushed robin eggs into batter. Spread evenly in prepared pan. Bake 28-32 minutes until toothpick inserted in center comes out clean. Cool completely in pan.
07 - Whisk powdered sugar with 1 tablespoon milk, adding more milk as needed to achieve thick but pourable consistency. Divide glaze and tint with food coloring if desired.
08 - Drizzle or spread colored glaze over cooled cake. Garnish with whole robin eggs. Allow glaze to set before slicing into 12 pieces.