Preheat your oven to 325 degrees Fahrenheit. Line a 9x12 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 gluten free flour, granulated sugar, baking powder and salt, until well combined. In a large bowl, combine the cornstarch and water with a spoon until it is thin and watery. It will be thick and tacky at first. Once combined, whisk in the milk of choice, canola or vegetable oil and vanilla. Once combined, pour the dry ingredients into the wet, a little at a time. It will be thick so you will probably need to use a spatula to combine.
Spread the batter into the prepared baking dish. You will need to smooth it out. It’s a little difficult because it’s so thick.
In a medium bowl, combine the gluten free flour for the crumb topping, the light brown sugar and the cinnamon. Pour in the melted butter and stir with a spatula until large crumbs form. Sprinkle all the crumbs over the batter.
Bake in the center rack for 30 min. Take out and let cool on the pan. Once cool, lift the crumb cake out using the overhang from the parchment paper. Dust with the confectioners’ sugar and cut in squares. Enjoy!
Storing: Wrap extra squares individually in foil and then pop them in a zip top bag at room temperature for up tp 3 days.
Freezing: Wrap extra squares individually in foil and then pop them in a zip top bag and freeze for up to 30 days. To defrost, leave the squares wrapped in foil at room temperature until soft. It might take 1-2 hours.
For vegan: Use vegan baking sticks, a non dairy milk and a gluten free flour that is free from dairy.