Position an oven rack in the middle of the oven and preheat the oven to 350F [180C]. Grease a 10 in [25 cm] or a 9 in [23 cm] square baking pan and line with a parchment sling.
In a large bowl, whisk together the melted butter, granulated sugar, eggs, sour cream, buttermilk, almond flour, vegetable oil, vanilla, baking powder, salt, and baking soda. Add the flour and use a spatula to stir it into the batter. Switch back to a whisk and whisk the batter to eliminate any remaining flour lumps, about 10 seconds.
Pour the batter into the prepared pan and use an offset spatula to smooth the top. Tap the pan gently on the counter twice to help get rid of any air bubbles.
Bake for 42 to 50 minutes, rotating the pan halfway through, until the cake is golden brown and a wooden skewer or toothpick inserted into the center comes out clean.
Transfer the pan to a wire rack and let cool for 20 minutes. Use the parchment sling to remove the cake from the pan, move to the wire rack, and then let cool completely. Dust the cake with confectioners' sugar. Cut the cake into squares and serve. The cake can be stored in an airtight container at room temperature for up to 3 days.