Start by preheating the oven to 350°F. For easier removal of the bars, lightly grease the bottom of a 9x13-inch baking pan and line it with parchment paper.
Combine the flour, baking powder, and baking soda in a medium mixing bowl and set aside.
In another mixing bowl, cream the butter, granulated sugar, and brown sugar together using a hand mixer on medium-high speed. It will look a little gritty.
Mix in the eggs, vanilla, and salt on medium speed until just incorporated. Do not overmix.
Gradually add the flour mixture and mix until you get a cookie dough consistency.
Fold in 1½ cups of the chopped mini Cadbury eggs until evenly distributed in the dough. Set the remaining mini Cadbury eggs aside for topping.
Spread and press the cookie dough down into an even layer in the prepared baking pan. Sprinkle on the remaining eggs and lightly press them in so they stay in place.
Bake for 30 to 35 minutes or until a toothpick inserted in the center comes out clean.
After baking, leave the bars in the pan and set them on a wire rack. Allow it to cool at room temperature before cutting it into squares and serving.