Preheat your oven to 325 degrees Fahrenheit. Line a 9x13 inch baking dish with parchment paper leaving a one inch overhang on either side. Lightly grease the parchment paper with vegetable oil. Set aside.

In a medium bowl, whisk together the 1.5 cups gluten free flour, ½ cup granulated sugar, 2.5 teaspoons baking powder and ½ teaspoon kosher salt. Set aside.

In a large bowl, using a spoon, mix together the 2 heaping tablespoons cornstarch and 3 tablespoons water until thin and watery. It will be thick and tacky at first.

Into the cornstarch water, whisk in the ¾ cup milk, the 2 tablespoons vegetable oil and the 2 teaspoons pure vanilla extract until combined.

Pour the dry ingredients into the wet a little bit at a time. Once it gets too thick to whisk, switch to using a spatula.

Using a butter knife or an icing spatula, spread the batter into the prepared baking dish and smooth it out. It’s a little difficult because the batter is so thick. The layer will be thin.

Cut the 1 cup butter or vegan baking stick into pieces and place in a heat safe bowl. Microwave in 20 seconds increments, stirring after each time until melted. Set it aside to cool.

In a large bowl, whisk together the remaining 2.5 cups of gluten free flour, the 1 cup light brown sugar and the 1.5 teaspoons ground cinnamon. Pour in the melted butter and stir with a spatula until large crumbs form. You came also use clean hands to form the crumbs if easier.

Sprinkle the crumbs all over the batter. There are lots of crumbs, make sure to use them all and cover every inch of batter. The crumbs will pile on top of one another; that's normal.

Bake the cake in the center rack of the 325 degree oven for 30 minutes or until a skewer inserted in the center of the cake comes out clean.

Let the cake cool in the pan completely before removing from the pan. Once cool, use the parchment overhang to lift the cake out.Make sure the cake is completely cool prior to dusting, otherwise the sugar will melt.

Dust with the confectioners’ sugar and cut in squares. Enjoy!