Heat the oven to 350°F. Line an 8 1/2 x 4 1/2-inch loaf pan with parchment paper.
Melt the butter in a small saucepan over medium heat and cook until the milk solids turn golden brown and smell nutty. Remove from the heat and cool for 8 minutes.
Whisk the almond flour, coconut flour, baking powder, sea salt, and cinnamon together. In another bowl, whisk the eggs, sour cream, allulose, vanilla, and browned butter until smooth.
Stir the wet mixture into the dry ingredients, then fold in 1/2 cup of the pecans. Spread the batter in the loaf pan and scatter the remaining pecans over the top.
Bake for 42 to 50 minutes, until the center is set and the top is deeply golden. Loosely cover with foil near the end if the pecans are browning too quickly.
Cool in the pan for 20 minutes, then transfer to a rack and cool completely. The crumb becomes richer and more tender as the browned butter settles into the loaf.