Preheat oven to 375° F.
Whisk together flour, baking soda, cinnamon, salt, and cream of tartar. Set aside.
In a large bowl using a hand-held mixer or stand mixer fitted with a paddle attachment, cream butter and sugar for 2 minutes until light and fluffy.
Add egg and vanilla and mix until incorporated.
Slowly add the dry ingredients to the wet and mix until just combined.
Using a cookie scoop place 6 rounded Tablespoons on an ungreased cookie sheet lined with parchment paper or a silicone baking mat. Gently press dough down into discs (about ¼” thick). Top with approximately 1 teaspoon Biscoff Spread then another disc of cookie dough. Pinch the seam with your fingers to seal in the Biscoff Spread. Roll in cinnamon sugar.
Bake for 10-12 minutes. Allow the cookies to cool on the baking sheet for 4 minutes and transfer to a wire rack to cool completely. Sprinkle each cookie with more cinnamon sugar if desired.