Preheat oven to 350°F. Line a baking sheet with parchment paper and set aside.
Whisk together flour, baking soda, salt, and cream of tartar in a medium bowl. Place butter and 1 ¾ cup sugar in the bowl of a stand mixer fitted with a paddle attachment. Cream on medium-high speed until very light and fluffy, 3-5 minutes.
Add the egg, egg yolk, and vanilla and mix until combined, scraping down sides of bowl as needed. Mix in dry ingredients until just combined.
Put remaining ½ cup sugar in a wide, shallow bowl. Scoop the dough into walnut-sized pieces (about 2 Tablespoons each), roll into smooth balls and toss to coat in sugar. Store on a baking sheet in the fridge.
Evenly arrange 6 balls on the prepared baking sheet. Bake for 12-14 minutes, or until firm on the sides and light golden brown. Let cool a few minutes on the baking sheet, then transfer to a wire rack to cool completely.
Repeat baking cookies, 6 at a time, until all of the dough balls have been baked.
When the cookies are cool, make Vanilla Fudge buttercream. Scoop Challenge Vanilla Fudge Snack Spread into the bowl of a stand mixer fitted with a paddle attachment. Beat 30 seconds to smooth out. Add all of powdered sugar, attach a splash guard, and slowly mix until it comes together (mixture will resemble breadcrumbs at first). Increase speed and beat until buttercream is stiff and smooth. Transfer to a piping bag fitting with a ribbon tip.
Place two small candy eyes on a cookie, securing with a small dab of buttercream. Pipe Vanilla Fudge buttercream in angled strips across the cookie to look like a mummy. Repeat decorating remaining cookies. Store in the refrigerator for best results.