Preheat your oven to 350°F. Grease and line the bottom of a 9x9-inch baking pan with parchment paper. In a bowl, whisk together the almond flour, coconut flour, baking powder, and salt.
In a large bowl, beat the egg yolks with 1/3 cup of the erythritol until thick and pale, about 2 minutes. Stir in the melted butter and vanilla extract. Fold in the flour mixture until combined.
In a separate clean bowl, beat the egg whites with the remaining 1/3 cup erythritol until stiff peaks form. Gently fold the egg whites into the batter in three additions, taking care not to deflate them. Pour into the prepared pan.
Bake for 22 to 26 minutes until the cake is golden and a toothpick comes out clean. Let the cake cool in the pan for 15 minutes.
While the cake cools, whisk together the heavy cream, coconut cream, almond milk, 3 tbsp powdered erythritol, and vanilla extract for the soak. Poke holes all over the warm cake using a fork or skewer, spacing them about an inch apart.
Slowly pour the three milks mixture over the cake, letting it absorb before adding more. Refrigerate for at least 4 hours, or ideally overnight.
Before serving, whip the 1 1/2 cups heavy cream with 3 tbsp powdered erythritol and vanilla extract to soft, billowy peaks. Spread it over the soaked cake in a thick, generous layer.
Dust with a light layer of ground cinnamon, slice, and serve cold. Each bite should be incredibly moist and creamy.
