In the bowl of a stand mixer fitted with the paddle attachment, beat butter and ½ cup (100 grams) sugar at medium speed until creamy, 3 to 4 minutes, stopping to scrape paddle and bottom and sides of bowl. Add egg yolk, beating well until combined. Add condensed milk and vanilla, beating just until combined.
In a medium bowl, whisk together flour, cornstarch, baking powder, salt, cream of tartar, and baking soda. With mixer on low speed, gradually add flour mixture to butter mixture, beating just until combined.
Line rimmed baking sheets with parchment paper.
Using a 1½-tablespoon spring-loaded scoop, scoop dough (about 28 grams each), and roll into balls. Place on prepared pans, and freeze for 15 minutes.
Preheat oven to 350°F (180°C).
In a small bowl, place remaining ¼ cup (50 grams) sugar. Carefully roll each frozen dough ball in sugar to coat, and place 2 inches apart on prepared pans.
Bake until puffed and edges are lightly browned, 12 to 14 minutes. Let cool on pans for 5 minutes. Remove from pans, and let cool completely on wire racks. Store in an airtight container for up to 3 days.