Preheat your oven to 350°F (175°C) and line a 9x13-inch baking pan with parchment paper.

Combine ½ cup of salted butter, ¾ cup of brown sugar, and ¼ cup of granulated sugar in a large mixing bowl. Beat until smooth and creamy for 2-3 minutes.

Add the egg yolk and 1 teaspoon of vanilla extract to the butter mixture and mix on low speed until fully combined.

Gradually mix in 2 cups of all-purpose flour and 1 teaspoon of baking powder until just combined.

Spread the cookie dough evenly into the prepared baking pan, smoothing out the surface.

Bake for 20-25 minutes or until lightly golden. Insert a toothpick to check for doneness.

Cool the bars completely on a wire rack for about 30 minutes.

Prepare the buttercream frosting by beating together 1 cup of unsalted butter and 4 cups of powdered sugar until smooth and fluffy.

Optionally, mix in food coloring to achieve pastel hues.

Spread the frosting evenly over the cooled cookie base.

Crush the Cadbury Mini Eggs and sprinkle them over the frosted bars.

Cut the bars into squares and serve on a festive platter.