Preheat the oven to 325°F and grease a 9-inch square baking pan.
In a large bowl, whisk together the oil, sweetener, eggs, and vanilla extract.Add the dry ingredients—the almond flour, cocoa powder, baking powder, and salt—and whisk until well-combined. Stir in the chopped nuts, if using.
Bake for 20 minutes, or until the edges are set but the center is still a little soft to the touch. Remove from the oven and let cool completely in the pan. Then cut into 16 squares.