Preheat the oven to 350°F. Line an 8×8 baking pan with aluminum foil or parchment paper, and set aside.
In a medium bowl, beat together the butter and sugar until light and fluffy. Mix in the egg and vanilla until fully combined. Slowly mix in the flour, baking powder and cornstarch until fully incorporated. Fold in the mini eggs.
Press the mixture into the prepared baking pan.
Place in preheated oven for 20-25 minutes, or until the top looks firm and golden.
Remove from oven allow to cool in the pan completely before removing to cut into squares.