Start by preheating your oven to 350°F (175°C). Line a baking sheet with parchment paper to prepare for your cookies.
In a large mixing bowl, combine the white rice flour, buckwheat flour, corn starch (or tapioca starch), cane sugar, and salt. Whisk them together until well blended.
In another bowl, use a hand mixer or stand mixer to cream the softened dairy-free butter and vanilla extract until the mixture is light and fluffy.
Gradually add the dry ingredients to the creamed butter mixture. If the dough feels too crumbly, add the optional tablespoon of water to help it come together. Mix until a soft dough forms.
Using a cookie scoop or spoon, portion out small amounts of dough and roll them into balls. Place the balls on the prepared baking sheet, spacing them a couple of inches apart.
Using your thumb or the back of a spoon, gently press down in the center of each dough ball to create an indentation. This is where your filling will go!
Spoon a small amount of blueberry or apricot jam into each thumbprint. Be careful not to overfill, as the jam can bubble over while baking.
Place the baking sheet in the preheated oven and bake for 12-15 minutes, or until the edges are lightly golden. Keep an eye on them to prevent burning.
Once baked, remove the cookies from the oven and let them cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely. After they have cooled, you can add chocolate or lemon curd into the thumbprints if desired.
