Preheat the oven to 350 degrees. Grease and flour a 10 or 12 cup Bundt pan.
In a large mixing bowl, use an electric mixer to cream together the butter and sugar until well combined.
Add the eggs, one at a time, and beat until combined. Then add the vanilla and sour cream and beat until completely incorporated.
In a medium mixing bowl, whisk together the gluten free flour, baking powder, baking soda, and salt.
Add ⅓ of the flour mixture to the batter, and with the mixer on low, beat until just combined. Continue to add ⅓ of the flour and mixing on low until all the flour has been incorporated.
Use a rubber spatula to scrape down the sides of the bowl and make sure the batter is completely combined.
In a small mixing bowl, add the brown sugar, gluten free flour, cinnamon, salt, and cold butter pieces. Use a fork, or your fingers, to press the streusel ingredients together until it forms crumbs.
Sprinkle ½ cup of the streusel in the bottom of the Bundt pan. Spoon half the batter over the streusel and spread evenly with a knife. Sprinkle the rest of the streusel on top of the batter. Then spoon the remaining batter over the streusel and spread it out.
Bake for 50 to 55 minutes, until a wooden pick inserted in the middle comes out clean.
Allow the cake to cool on a wire rack for at least 30 minutes. Then invert the cake onto a wire rack to cool completely.
Whisk the powdered sugar and milk together to form the glaze. Stir in additional powdered sugar or milk to achieve the desired consistency. Use a spoon to drizzle the glaze over the cake.
Notes
Generously grease and flour the Bundt pan. This will keep the edges of the cake from sticking to the pan.
Check the ingredients of your gluten free flour mix for xanthan gum. If your flour mix doesn’t contain it, try adding ½ teaspoon to the recipe. I used Bob’s Red Mill Gluten Free 1 to 1 Baking Flour which has xanthan gum in it.
Use a butter knife to spread the batter in the Bundt pan. The smaller knife makes it easier to spread the batter into all the crevices of the pan.