Preheat the oven to 180°C (356°F).
Finely chop the hazelnuts.
In a bowl, combine cake flour, cocoa powder, baking powder, and salt.
Add cold butter and mix until it becomes small crumbles.
Add allulose, egg, and soy milk, and knead into a dough.
Mix in the hazelnuts and dark chocolate.
Shape the dough into triangles.
Bake in the oven for 20–25 minutes.