Preheat the oven to 350 degrees Fahrenheit. Line a 9x13 baking pan with parchment paper, and spray with nonstick spray. Set it aside.
Whisk together the butter and sugars in a large bowl until fully combined.
Add in the eggs, one at a time, and the vanilla extract and mix until combined.
In a separate medium sized bowl, whisk together the flour, baking powder, and salt.
Add the dry ingredients to the wet ingredients, and fold them together, just until combined.
Pour in 1/2 cup of the Cadbury mini eggs , and fold them into the mixture.
Pour the batter into the prepared pan and spread evenly using a rubber spatula.
Sprinkle another 1/2 cup of Cadbury eggs over the top of the batter.
Bake for about 28 to 30 minutes, or until an inserted toothpick comes out clean.
Remove from the oven and press the last 1/2 cup of Cadbury mini eggs into the top of the cookie bars.
Allow to cool slightly, then remove from the pan and slice them up.