Prepare the sweetened condensed coconut milk by combining two tablespoons of the coconut milk with the cornstarch in a small bowl and stirring until well combined. In a microwave-safe bowl (or a small saucepan), combine the remaining coconut milk, the cornstarch/coconut milk mixture, and 1 cup of sugar. Stir and then heat in the microwave for 44 seconds. Stir and repeat several times until the mixture has thickened to your liking. It should resemble sweetened condensed milk. Stir in the vanilla and set aside to cool.
Heat your oven to 350F. Prepare your 9 x 13 baking pan by lining it with foil, making sure the foil extends past each edge of the pan. This recipe expands as it bakes so the foil helps prevent a mess in the oven. Then spray the foil with cooking spray.
Make the chocolate layer by stirring together softened 1/2 cup vegan butter with the cake mix and 1/4 cup of the condensed milk. This is a sturdy dough. Spread the chocolate cake dough in the bottom of the prepared pan.
Rinse out the bowl. Make the vanilla layer by stirring together softened 1/2 cup vegan butter with the vanilla or yellow cake mix and 1/4 cup of the condensed milk. Spread the vanilla cake dough over the chocolate layer.
Make the strawberry layer by stirring together softened 1/2 cup vegan butter with the strawberry cake mix and 1/4 cup of the condensed milk. Spread the strawberry cake dough over the chocolate layer.
Pour the remaining condensed milk over the top, being sure to cover each area of the top layer equally. Be sure the foil is standing tall on each side of the pan before placing it in the oven.
Bake for 40 to 45 minutes until a tester inserted in the center comes out clean.
Remove from oven and set aside to cool. Trim the edges, cut the center into bars and serve.
* Some commercial cake mixes are what we call "accidentally vegan." There's a lot of ingredients listed (usually something we keep to a minimum), but just look for the ones that don't list animal products such as milk, eggs, animal shortening, etc).