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, about 3 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, 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, or until a toothpick inserted in the center comes out clean. Let cool completely in the pan.
06 - Whisk powdered sugar with 1 tbsp milk, adding more 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. Let set before slicing into squares.