Adjust toaster oven or oven rack to middle position and heat oven to 350 ̊F. Line a small rimmed baking sheet (perhaps an eight sheet pan?) with parchment paper.
Whisk flour, baking soda, and salt together in a small bowl.
Using a rubber spatula, mash butter and sugar together in separate bowl until well combined and lightened in color. Add egg yolk and vanilla and mix until combined. Stir in flour mixture until just combined, then stir in chocolate chips.
Divide dough into 2, and put each part in a little heap on the prepared baking sheet, spaced about 2 inches apart. Bake until edges of cookies are set and beginning to brown but centers are still soft and puffy, 8 to 12 minutes.