01 - Preheat oven to 350°F. Line a baking sheet with parchment paper.
02 - In a medium bowl, whisk together almond flour, coconut flour, powdered erythritol, salt, and baking soda until well blended.
03 - In a large bowl, beat softened butter with an electric mixer until fluffy. Add egg and vanilla extract, mixing until fully incorporated.
04 - Gradually add dry ingredients to wet mixture, mixing until soft dough forms.
05 - Divide dough evenly between two bowls. To one bowl, add cocoa powder, hazelnut butter, and 1 tablespoon powdered erythritol. Mix thoroughly to create chocolate hazelnut dough.
06 - Place both doughs between separate sheets of parchment paper. Roll each into ¼-inch thick rectangles of similar size.
07 - Remove top parchment sheets. Gently place chocolate hazelnut dough over plain dough. Roll up tightly jelly-roll style. Wrap in parchment and freeze for 20 minutes until firm.
08 - Using a sharp knife, slice log into ¼-inch thick cookies. Arrange on prepared baking sheet, spacing 1 inch apart.
09 - Bake for 10 to 12 minutes until edges are just golden. Cool completely on baking sheet before serving.