Preheat the oven to 325 degrees. Grease a non-stick bundt pan generously with butter.
In a small bowl combine the flour, baking powder, baking soda, sea salt, and xanthan gum. Stir to combine.
In a large glass bowl, beat butter until fluffy with an electric hand mixer, then add sugar and mix well.
Add the eggs to the bowl, and mix well until batter is smooth and creamy, scraping the sides of the bowl as needed.
Add HALF of the flour mixture and gently mix with a spatula until almost all combined. It is important to mix gently as this is incorporating air and fluffiness to the cake. Don't over mix.
Add the sour cream and vanilla to batter. Then gently mix in.
Add the remaining HALF of flour mixture, and continue to GENTLY mix and fold, making sure that all the flour is incorporated.
Layer HALF the batter into the greased bundt pan.
Next, sprinkle your mixture of brown sugar and 2 tsp of cinnamon over the first layer of batter.
Then gently spread the other half of batter on top of the cinnamon/brown sugar layer. Then bake at 325 degrees for 36-38 minutes or until lightly browned on top and a toothpick comes out clean.
Let cool on cooling rack for 10 minutes. Then using a small spatula gently move up and down around the edges of the pan to make sure the cake isn't stuck to the pan. Also, make sure to do the same for the center of the cake. You may need to use a knife for this part, just be careful not to damage you cake pan.
Using your cooling rack, flip over the cake onto the cooling rack.
Mix powdered sugar, cinnamon, and milk to make the glaze. Then gently baste it onto the top and sides of cake. Then continue to let cool on the wire rack.
Then slice and serve! This pairs so nicely with homemade vanilla ice cream.