Preheat the oven to 350°F. Line a 8.5x4.5 loaf pan with sling of parchment paper. A 9x5 pan or 9.25x5.25 pan will also work but produce a shorter loaf. For a larger pan, adjust your baking time, checking for doneness at 50 minutes.

In a medium bowl, sift together oat flour, almond flour, sweet rice flour, tapioca flour, xanthan gum, baking powder, and salt. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment or a large mixing bowl with a hand mixer, cream the butter and sugar on medium speed until light and fluffy, 2-3 minutes, scraping down the sides as needed. The butter should turn pale in color, but still have structure. Take care not to over-cream to the point where the butter begins to get warm and soften. Mix in the vanilla extract.

Add the eggs, one at a time, mixing until just combined and scraping down the sides after each addition. The mixture may look somewhat curdled at this point, but that's okay!

Add the dry ingredients in two additions, alternating with the sour cream, scraping down the sides after each addition and mixing until just combined. Pour the batter into the prepared baking pan and smooth out the top with an offset spatula. Optional: for the perfect split down the center, use a knife or pastry cutter to make a ⅛ to ¼-inch deep slice down the middle. Pipe a thin line of butter down the slice.

Bake on the middle rack for 65-70 minutes, until a toothpick or skewer inserted into the center comes out clean or with just a few crumbs. The cake should be deep golden brown on the outside. The center of the dome should spring back when gently touched. If you notice the top beginning to get too dark, tent it loosely with foil for the last 5-10 minutes.

Remove from oven and let cool in the pan for 10 minutes before using a knife to loosen the short edges and transferring the loaf to a wire rack to cool completely before cutting, about 1.5-2 hours. Listen, I, too, have no patience, so feel free to cut it sooner, but just know that the crumb will not fully set until it cools, so it may seem more dense (yet still delicious!)