Preheat the oven to 350 F and grease a 9-inch square baking pan. For the crumb topping, stir together 3/4 cup gluten free 1-to-1 baking flour, brown sugar, cinnamon, and 1/4 teaspoon salt. Pour in the melted butter and mix with a fork until large, clumpy crumbs form. Set aside.
For the cake, whisk together 1 3/4 cups gluten free 1-to-1 baking flour, baking powder, baking soda, and 1/4 teaspoon salt in a medium bowl.
In a large bowl, beat the softened butter and granulated sugar on medium-high speed for 2 to 3 minutes until light and fluffy. Beat in the eggs one at a time, then add the vanilla extract. Add the flour mixture in two additions, alternating with the sour cream, mixing on low speed just until combined.
Spread the batter evenly into the prepared pan. Break the crumb topping into large pieces and scatter generously over the batter. Bake for 35 to 40 minutes until a toothpick inserted in the center of the cake comes out clean.
Let the cake cool in the pan for 20 minutes. Whisk together the powdered sugar and milk until smooth, then drizzle over the top of the warm cake. Serve warm or at room temperature.