This gluten free crumb cake has the perfect ratio of crumb to cake. The cake is tender and light, but it's really all about those big and crunchy crumbs. Bonus? It can be made with ingredients you probably already have in your gluten free pantry!
Preheat the oven to 350° F and spray a 9 by 13-inch baking pan with nonstick spray (or lightly grease with butter or shortening), dust it with gluten free flour, and tap out the excess flour.
Cream the butter and sugar together until light and fluffy. Add the egg, sour cream, and vanilla extract and mix well.
Sift or whisk together the flour, baking powder, and salt into a small bowl. With the mixer on low speed, alternate adding the dry ingredients with the milk in three batches, beginning and ending with the flour mixture.
Spread the mixture into prepared pan in an even layer to the edges. The batter will be thin, only about one inch high. Set aside.
FOR THE CRUMB TOPPING:
In a medium bowl, whisk together the flour, brown sugar, and cinnamon until well combined. Add the melted butter and stir until it comes together into a thick paste.
Using your hands, scrunch together large handfuls of it and then break it apart right over the pan, sprinkling large and small chunks here and there to cover the entire surface of the cake batter.
Bake the cake for about 30 minutes, or until set in the center and the cake springs back when lightly touched on the top. If the center is still not quite done, reduce the temperature to 325° F and continue to bake at 5-minute intervals until completely done and top isn't too brown.
Remove from the oven and, if desired, sprinkle with powdered sugar. Allow to cool and slice and serve.