Prepare the crust by whisking together sweet rice flour, millet flour, gluten-free oat flour, tapioca flour, and kosher salt. Cut in cold unsalted butter, then gradually add ice water. Shape into a disc, wrap in plastic, and chill for 30 minutes.

Preheat your oven to 400°F (200°C) while the dough chills. Prepare tart pans with a light coat of cooking spray.

Roll out dough between two sheets of parchment paper to about ⅛ inch thick. Transfer circles into tart pans and trim excess dough, pricking bottoms to prevent bubbling.

Bake crusts for 12 minutes until just lightly cooked. They should be pale and starting to firm up.

In a mixing bowl, combine ricotta cheese, Greek yogurt, egg whites, minced garlic, and a pinch of salt. Stir until smooth and creamy, adjusting seasoning as desired.

Fill pre-baked crusts with ricotta mixture. Decorate with black olives for Jack Skellington's features, using julienned olive strips for embellishment.

Return decorated tarts to the oven and bake for 20-25 minutes until filling is set and lightly browned.

Allow tarts to cool slightly before transferring to a serving platter. Serve warm and enjoy!