Preheat the oven to 350°F. Lightly grease and line an 8x8 baking pan with parchment paper. You can skip the greasing, but it just helps hold the parchment paper in place when spreading the thick batter.

In a medium bowl, sift together the sweet rice flour, oat flour, millet flour, ground cardamom, xanthan gum, baking soda, baking powder and salt. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment or a bowl with a hand mixer, cream together the butter and sugar over medium speed until light and fluffy, about 3-5 minutes.

With the mixer on low-speed, add the eggs one at a time, mixing after each one.

Add the vanilla extract, mixing until just combined.

Keeping the speed on low, slowly add the dry ingredients in two batches, adding the sour cream in between, mixing until just combined. Your batter will be very thick.

Pour the batter into the prepared baking pan and use a rubber spatula to spread it into an even layer.

Sprinkle the cranberries evenly over the top.

In a separate medium bowl, make the crumb topping. Mix the oat flour, sweet rice flour, tapioca flour, light brown sugar, and ground cardamom together. Add the melted butter and mix until it comes together when pinched with your fingers.

Top the cranberries with the crumb topping, creating some small and some large crumbles by pinching the crumbs together.

Bake for 45-55 minutes until the edges of the cake springs back when pressed and a toothpick comes out dry or with a few moist crumbs.

Remove from oven and let cool for at least 10 minutes in the pan. Use the parchment paper to lift the cake out of the pan to a cooling rack to cool.

Once cool to the touch, use a sifter to sprinkle powdered sugar over the top, cut into 9 slices and serve.