Preheat oven to 400 degrees.
In a large bowl, add flour, baking powder, xanthan gum, sea salt, sugar, and oats. Stir well.
Add cold butter pieces and cut into dry ingredients using a pastry blender.
Add milk and stir gently with a fork until all ingredients are mixed. Let stand 2-3 minutes.
On generously floured(same all purpose gluten free flour) parchment paper, place half of the dough. Flour the top and your rolling pin and gently roll dough into about a 9.5x9.5 circle. Carefully place the parchment paper with dough rolled out onto a baking sheet. Repeat with the other half of the dough.
Melt 3 TBSP of butter and spread onto dough using a basting brush. Mix the sugar and cinnamon together in a small bowl. GENEROUSLY sprinkle cinnamon/sugar mixture on top of the melted butter on the dough. (I always have a container of cinnamon/sugar mixed together so that it is easier to sprinkle). I never measure the exact amount of cinnamon/sugar. But trust me you want it to be GENEROUSLY sprinkled. I don't think you'll need all the cinnamon/sugar mixture, this is just how much I mix up at a time.
Bake in the oven at 400 degrees for 15-17 minutes. I like to switch my baking sheets half way through for even baking in my oven. They are done when the edges are lightly browned.
Cut into wedge-shaped slices and enjoy!