Preheat oven to 325°F (165°C). Grease a loaf pan or bundt pan.
Whisk together almond flour, coconut flour, baking powder, and salt.
In a separate bowl, beat butter and brown sugar substitute until light and fluffy, 3–4 minutes.
Beat in eggs one at a time, mixing well after each addition.
Mix in buttermilk, vanilla, and almond extract.
Fold dry ingredients into the wet mixture until just combined.
Pour into the prepared pan and bake 50–55 minutes, until a toothpick comes out clean.
Cool in the pan 15 minutes, then turn out onto a wire rack to cool completely.