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 combined.
03 - Beat butter and sugar in a large bowl until light and fluffy, approximately 3-4 minutes. Add eggs one at a time, beating well after each addition. Mix in vanilla extract.
04 - Add one-third of the dry ingredients to the butter mixture, mixing gently. Add half the buttermilk and mix. Repeat with another third of dry ingredients, remaining buttermilk, then finish with the last of the dry mix. Mix just until combined.
05 - Fold in the crushed robin eggs until evenly distributed. Spread batter in prepared pan. Bake for 28-32 minutes until a toothpick inserted in center comes out clean. Cool completely in pan.
06 - Whisk powdered sugar with 1 tablespoon milk, adding more as needed to achieve thick but pourable consistency. Divide glaze and tint with food colors if desired. Drizzle or spread over cooled cake.
07 - Top with whole robin eggs while glaze is still tacky. Allow glaze to set completely before slicing into squares.