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