01 - Preheat the oven to 350°F. Line a baking sheet with parchment paper.
02 - In a large bowl, whisk together the hazelnut flour, almond flour, cocoa powder, erythritol, salt, and baking powder until well combined.
03 - In a separate bowl, whisk the egg, melted butter, and vanilla extract until smooth and fully incorporated.
04 - Add the wet ingredients to the dry ingredients. Mix until a soft, uniform dough forms.
05 - Scoop tablespoon-sized portions of dough and roll into balls. Place on the prepared baking sheet, spacing them about 2 inches apart.
06 - Use your thumb or the back of a teaspoon to press a deep indentation in the center of each cookie ball.
07 - Bake for 12-15 minutes, until cookies are set but still slightly soft. Remove from oven and reinforce indentations if needed. Let cool on baking sheet.
08 - While cookies cool, melt chocolate chips, heavy cream, and butter together in a microwave-safe bowl in 30-second intervals, stirring until smooth and glossy.
09 - Spoon approximately 1 teaspoon of chocolate filling into the center of each cooled cookie.
10 - Sprinkle with chopped toasted hazelnuts if desired. Allow filling to set completely before serving.