Healthy Thumbprint Cookies

In a medium bowl, whisk together the flour, cornstarch, baking powder, and sea salt. In a separate bowl, whisk together the butter, egg white, vanilla, and milk. Stir in the coconut sugar. Add in the flour mixture, stirring until incorporated. Chill the cookie dough for at least 30 minutes. Preheat the oven to 350°F, and line a baking sheet with a silicone baking mat or parchment paper. Roll the cookie dough into about 24 small spheres, and place them on the prepared baking sheet. Using your thumb to make an indentation in the center of each, and use a small spoon to place the jam into the indentations. Bake at 350°F for 10 minutes. Cool on the baking sheet for 10 minutes before carefully transferring to a wire rack to cool completely.