01 - Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper to prevent sticking.
02 - In a medium bowl, whisk together the almond flour, coconut flour, erythritol, baking powder, and salt until uniformly mixed.
03 - In a large bowl, use a hand mixer to cream the softened butter until fluffy. Beat in the egg and vanilla extract until the mixture is smooth and well incorporated.
04 - Gradually add the dry ingredient mixture into the wet mixture, mixing on low speed until a cohesive soft dough forms.
05 - In a small separate bowl, combine the walnut butter, powdered erythritol, and melted butter. Stir until the mixture is completely smooth and consistent.
06 - Divide the cookie dough in half. Place one half between two sheets of parchment paper and roll or press it into a rectangle approximately 8x6 inches.
07 - Spread half of the walnut butter mixture evenly over the surface of the dough rectangle. Roll the dough up tightly from the longer edge to form a log. Repeat this process with the second half of the dough and remaining filling.
08 - Place the dough logs in the freezer for 10 minutes. This firms them up, making them easier to slice cleanly.
09 - Remove the logs from the freezer and slice them into cookies roughly 1/2-inch thick. Arrange the slices on the prepared baking sheets.
10 - Bake for 12 to 14 minutes, or until the edges are golden brown. Let the cookies cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.