Preheat the oven to 325F. Line a 8x8-inch square baking dish with parchment paper. Spray with non-stick cooking spray. Set aside.
In a medium mixing bowl, add the flour, cornstarch, baking powder, and salt. Whisk to combine. Set aside.
In a large mixing bowl, add the melted butter and brown sugar. Mix until combined and smooth.
Add the egg, egg yolk, and vanilla extract. Mix until combined.
Gradually add in the dry ingredients, mixing until no more streaks of flour remain.
Roughly chop 3/4 cup of the cadbury mini eggs, leaving the remaining 1/4 cup for later.
Stir in the chopped mini eggs until thoroughly distributed.
Pour the blondie batter into the prepared baking dish. Spread with a spatula or knife to smooth out the top. Top with the remaining whole mini eggs.
Bake at 325F for 25-29 minutes, until the edges are browned and a toothpick inserted into the center comes out with moist crumbs. If it comes out with dry crumbs, the blondies are over baked. If it comes out with wet batter, they need more time to bake.
Let the blondies cool in the pan for at least 15 minutes before removing to a cutting board to slice into squares.
