In a stand mixer using the paddle attachment, beat the vegan butter and sugar for 1 minute until smooth, creamy, and fluffy. You can also use a hand beater and beat ingredients into a mixing bowl.
Fold in the flour and optional ingredients if used, like vanilla and salt. Keep beating on the low-speed setting until well incorporated.
Scrape the sides of the bowl if needed to bring all the ingredients together. The thumbprint cookie dough should look crumbly but not dry. It is soft and holds well if pressed in your hand. If too wet, add more flour, 1 teaspoon at a time. If too dry, add 1 teaspoon of water.
Preheat the oven to 350°F (180°C). Line two cookie sheets with parchment paper. Oil the paper with coconut oil or vegan butter. Set aside.
Shape the dough into 1-inch balls and place each cookie dough ball into a bowl filled with sugar. Roll the ball to stick the sugar around it.
Place each cookie dough ball onto the prepared cookie sheets and use your thumb to press down the center of the cookie dough ball.
Fill each thumbprint with 1/2 teaspoon jam or a piece of dark chocolate.
Bake at 350°F (180°C) for 15-18 minutes in the center rack of the oven or until the thumbprint cookies are slightly golden brown.
Cool them down fully on a wire rack and cool down for an extra 20 minutes.
Store in a sealed cookie jar at room temperature for up to 1 week, or freeze and thaw the day before at room temperature.
Note 1: Any granulated sugar works well including white sugar, coconut sugar, brown sugar, or unrefined cane sugar.Note 2: This recipe works with the same amount of gluten-free all-purpose flour. The recipe won't work with coconut flour or almond flour.