01 - Preheat the oven to 350°F. Line a baking sheet with parchment paper.
02 - In a small saucepan, combine cranberries, water, erythritol, and lemon zest. Cook over medium heat, stirring frequently, until berries burst and the mixture thickens, approximately 5 to 7 minutes. Mash lightly and allow to cool.
03 - In a mixing bowl, beat softened butter and erythritol until light and fluffy. Add the egg and vanilla extract, mixing until fully incorporated.
04 - In a separate bowl, whisk together almond flour, coconut flour, baking powder, and salt.
05 - Gradually add the dry ingredients to the wet mixture, stirring until a soft dough forms.
06 - Scoop tablespoon-sized portions of dough onto the prepared baking sheet, spacing each about 2 inches apart.
07 - Create a small indentation in each cookie with your thumb. Spoon approximately 1/2 teaspoon of cranberry mixture into each indentation. Gently swirl using a toothpick or knife to create a marbled effect.
08 - Bake for 12 to 15 minutes or until edges turn lightly golden.
09 - Let cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.