Preheat your oven to 350°F (175°C).
Line your 9x13 inch baking pan with parchment paper, leaving some overhang.
In a large mixing bowl, whisk together the all-purpose flour, baking powder, and coarse salt. Set aside.
In another bowl, beat the unsalted butter and brown sugar together until light and fluffy (about 2-3 minutes).
Crack in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Gradually add the dry mixture to the wet mixture, stirring gently until just combined.
Fold in the chopped Cadbury Mini Eggs, miniature marshmallows, dark chocolate chips, and white chocolate.
Spread the cookie dough evenly into the prepared baking pan and smooth the top.
Sprinkle the remaining chopped Cadbury Mini Eggs, miniature marshmallows, dark chocolate chips, and white chocolate on top.
Bake for 25-30 minutes or until the edges are golden brown and a toothpick comes out clean.
Let cool in the pan for 10 minutes, then lift out using the parchment paper and cool completely before cutting into squares.