In a medium mixing bowl, combine the dry ingredients: almond flour, tigernut flour, tapioca flour, baking powder, and salt. Mix well.
In another bowl, cream the butter and granulated allulose together using an electric mixer, or a handheld whisk. Beat until light and creamy.
Add the eggs into the butter mixture, one at a time, incorporating them. Stir in the vanilla extract and ground cinnamon, blending until smooth.
Gradually fold in the dry ingredient mixture into the wet ingredients. Mix until just combined to keep the cake tender; avoid over-mixing. You will get a creamy, thick cake batter.
Stir in the chopped apples gently, ensuring they are evenly distributed throughout the batter.
Pour the batter into your prepared springform pan, smoothing the top with a spatula for an even bake. Sprinkle one teaspoon of granulated sweetener on top.
Bake in a preaheated oven at 350F (180C) for 40-45 minutes, or until the cake is golden brown, crispy on top, and firm to the touch. A toothpick inserted into the center should come out clean.
Allow the cake to cool in the pan for 10 minutes, then carefully remove it and gently transfer it to a serving platter. Be careful, using two spatulas, as the cake is still quite soft while warm.
Lightly dust with inulin powder or your chosen powder sweetener. Slice, serve, and savor every bite!