Preheat oven to 350°F and line a large baking sheet with parchment paper.
Cream together butter and sugar in a large bowl using a hand mixer until smooth.
Add vanilla extract and eggs, mixing until fully combined.
In a separate bowl, whisk salt, cream of tartar, baking soda, and flour together.
Gradually add the dry ingredients to the wet ingredients, mixing thoroughly and scraping the bowl sides as needed.
Fold in chopped Pop Tart pieces (reserve about ⅓ cup for topping) and rainbow sprinkles evenly.
Scoop 2 tablespoons of dough, roll into balls, and place on baking sheet spaced 2 inches apart.
If not glazing, press reserved Pop Tart pieces onto cookie tops before baking.
Bake cookies for 11 to 12 minutes until slightly golden but still soft.
Cool cookies on baking sheet for a few minutes, then transfer to a cooling rack.
Prepare glaze by mixing powdered sugar and heavy cream until smooth.
Once cookies are completely cool, dip tops into glaze and sprinkle with leftover Pop Tart pieces and sprinkles.