Preheat the oven to 350°F (175°C). Grease a 9-inch cake pan with a thin layer of butter, then layer on a circle of parchment paper to just cover the bottom of the pan, then grease the paper. Set aside.
Scatter the almonds in a single layer on a rimmed baking sheet. Toast until golden, tossing once during cooking, 4 to 6 minutes. Let cool.
In the bowl of a stand mixer fitted with the paddle attachment (or a large bowl, and using an electric hand mixer), combine 8 tablespoons of the butter and 1¼ cups of the powdered sweetener. Beat on medium speed, scraping down the sides of the bowl as necessary, until the mixture is pale yellow, light and fluffy, 1 to 2 minutes. Beat in the eggs one at a time until incorporated into a curdled-looking thin batter.
In a medium bowl, whisk together the almond flour, baking powder, and ½ teaspoon salt.
With the mixer at medium speed, add the flour mixture a little at a time to the egg mixture until combined. Mix in the almond and vanilla extracts.
Pour the batter into the prepared cake pan. Bake, rotating once during cooking, until the top is deep golden and springs back when lightly touched and a toothpick inserted into the center comes out clean, about 45 minutes.
Let cool on a wire rack for 15 minutes. Slide a small knife around the edges to release and invert the cake and place it on a wire rack set over a baking sheet to cool completely.
Melt the remaining 2 tablespoons butter in a small saucepan over medium heat. Whisk in the remaining 2 tablespoons of the powdered sweetener and a few teaspoons of water. Add the toasted almonds, toss to combine, and quickly pour over the top of the cake, using a small spatula or fork to evenly distribute.
Sprinkle with the remaining 1 tablespoon powdered sweetener and serve.