Preheat the oven to (350 °F). Butter a 20-cm (8-inch) springform pan and line the bottom with parchment paper. Set aside. In a bowl, combine the flour, baking powder, and salt. Set aside. In another bowl, beat the egg whites and cream of tartar with an electric mixer until soft peaks form. Gradually add 1/2 cup of the sugar, beating until stiff peaks form. Set aside. In a third bowl, beat the remaining sugar, butter, egg yolks, and vanilla with an electric mixer. At low speed, add the dry ingredients alternately with the oat milk. Stir a quarter of the meringue into the batter to lighten it. With a spatula, gently fold in the remaining meringue. Pour into the prepared pan. Bake for about 55 minutes or until a toothpick inserted in the center of the cake comes out clean. Unmould immediately and let completely cool on a wire rack.