01 - Preheat oven to 350°F. Grease and line an 8x8-inch baking pan with parchment paper.
02 - In a medium bowl, whisk together flour, baking powder, baking soda, and salt.
03 - In a large bowl, beat butter and sugar until light and fluffy. 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 wet mixture, mixing gently. Add half the buttermilk, mix, then another third of the dry ingredients. Add the remaining buttermilk, then finish with the last of the dry mix. Mix just until combined.
05 - Fold in the crushed robin eggs. Spread batter evenly in prepared pan. Bake for 28–32 minutes, or until a toothpick inserted in the center comes out clean. Let cool completely in the pan.
06 - Whisk together powdered sugar with 1 tablespoon milk, adding more milk as needed to achieve a thick but pourable consistency. Divide glaze and tint with food colors as desired.
07 - Drizzle or spread glaze over cake. Garnish with whole robin eggs. Let set before slicing.