01 - Preheat oven to 350°F. Grease an 8x8-inch baking pan and line with parchment paper.
02 - In a medium bowl, whisk together flour, baking powder, baking soda, and salt until well blended.
03 - In a large bowl, beat butter and sugar until light and fluffy, approximately 3-4 minutes. Add eggs one at a time, beating thoroughly 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 remaining dry mix. Stir just until combined.
05 - Fold in crushed robin eggs until evenly distributed. Spread batter into prepared pan. Bake for 28-32 minutes or until a toothpick inserted in center emerges clean. Cool completely in pan.
06 - Whisk powdered sugar with 1 tablespoon milk, adding additional milk as needed to achieve thick but pourable consistency. Divide glaze and tint with food coloring if desired.
07 - Drizzle or spread colored glaze over cooled cake. Garnish with whole robin eggs. Allow glaze to set before slicing into squares.