Combine almond flour, coconut flour, erythritol, baking powder and salt in a food processor. Pulse for 10 seconds until combined.
Add vanilla, stevia, egg and butter to the food processor and pulse until fully incorporated and dough is formed (will be very soft).
Place dough in saran wrap and refrigerate for at least 60 minutes. Line a rimmed baking sheet with parchment paper and set aside.
Preheat oven to 375°F and line a baking sheet with parchment paper.
While dough is chilling make the blackberry jam.
Place blackberries and lemon juice in a small saucepan set over medium heat. Bring to a boil, using a fork to crush berries as they soften.
Remove from heat and add chia seeds and stevia. Mix well and allow to cool.
Scoop cookie dough into tablespoon-sized balls and roll into smooth balls. Place 2 inches apart on baking sheet and use your thumb to gently make an indentation in the center of the cookie dough. Repeat with remaining dough.
Spoon cooled jam into each thumbprint, filling them to the top.
Bake for 9 (soft) to 11 (crunchy) minutes until edges begin to brown. Allow to cool for 5-10 minutes, then transfer to a cooling rack and allow to cool completely prior to serving.
Best stored in an air tight container in the fridge up to one week.