01 - Preheat your oven to 350°F. Grease an 8x8-inch baking pan and line with parchment paper.
02 - In a medium bowl, whisk together the flour, baking powder, baking soda, and salt until well combined.
03 - In a large bowl, beat the softened butter and granulated sugar until light and fluffy, about 3 minutes. Add eggs one at a time, beating well after each addition, then mix in the vanilla extract.
04 - Add one-third of the dry ingredients to the butter mixture, mixing gently. Add half the buttermilk, mix, then another third of the dry ingredients. Add remaining buttermilk, then finish with the remaining dry mix. Mix just until combined.
05 - Gently fold the crushed robin eggs into the batter. Spread evenly in the prepared pan and bake for 28-32 minutes, or until a toothpick inserted in the center comes out clean. Cool completely in the 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 coloring if desired.
07 - Drizzle or spread the glaze over the cooled cake. Garnish with whole robin eggs and let set before slicing into squares.