Preheat oven to 350F (180C). Lightly spray an 8-inch round or square baking pan with nonstick cooking spray.
In a large bowl, whisk together the almond flour, potato starch, coconut sugar, flaxseed meal, baking powder and (optional) salt.
Measure the water and add the vanilla to it. Pour into bowl with almond flour mixture and whisk until blended and smooth. Spread into prepared baking pan.
Bake in the preheated oven for 35 to 40 minutes until golden brown and a toothpick inserted near the center of the cake comes out with only moist crumbs attached.
Transfer cake to a cooling rack and cool completely in pan. Run a butter knife around edge of pan and invert cake on a plate. Turn right side up on cake plate.
If desired, lightly dust with confectioners' sugar just before serving.