Preheat oven to 350F and grease a 9-inch round pan with cooking spray.

In the bowl of a stand mixer fitted with the paddle attachment, cream the softened vegan butter and sugars on low-medium speed until smooth.

Add flax egg and vanilla extract, beat until combined and creamy.

Sift in the flour, cornstarch, baking powder, baking soda, and salt then mix on low until a soft cookie dough forms.

Add dark chocolate chips to the mixing bowl and stir until evenly distributed. I used regular morsels, but mini chocolate chips or chocolate chunks would be great too!

Transfer cookie dough to the prepared cake pan and press the dough into a smooth layer, making sure to get all the corners.

Bake for 20-25 minutes, until the top is golden brown and the center is firm. Remove from oven and let cool completely in the pan. Once cool, carefully remove from pan and place on a serving platter to decorate.

To make homemade vegan frosting, add softened vegan butter to the bowl of a stand mixer fitted with the paddle attachment, cream for about 1 minute, until smooth.

Slowly mix in sifted powdered sugar until fully incorporated, starting low and increasing speed. Add dairy free milk and vanilla, mix on medium-high for 1 minute, until creamy.