Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.

In a large mixing bowl, whisk together the chestnut flour, almond flour, baking soda, and salt.

In a separate bowl, cream together the softened butter and coconut sugar until light and fluffy.

Add the egg and vanilla extract to the butter mixture, mixing until smooth.

Gradually fold the dry ingredients into the wet ingredients, mixing until combined into a dough.

Roll the dough into small balls (about 1 inch in diameter) and place them on the prepared baking sheet.

Gently press your thumb into the center of each cookie to create an indent.

Fill the indents with about ½ teaspoon of fruit jam.

Bake for 10-12 minutes, or until the edges of the cookies are lightly golden.

Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.

Serve and enjoy your delicious chestnut flour thumbprint cookies!