Preheat oven to 350°F. Line two light-colored baking sheets with parchment paper.
Mix dry ingredients (2 min): In a medium bowl, whisk together the flour, baking soda, cornstarch, and salt.
2 1/2 cups 300g all-purpose flour (increase slightly for less spread), 1 teaspoon baking soda, 1 Tablespoon cornstarch, 1/2 teaspoon salt
Cream butter and sugar (3 min): In a large bowl using a handheld or stand mixer fitted with a paddle attachment, beat the butter, brown sugar, and granulated sugar until light and creamy, 2-3 minutes.
3/4 cup unsalted butter, 3/4 cup light brown sugar, 1/4 cup granulated sugar
Add egg and vanilla (1–2 min): Add the egg, egg yolk, and vanilla. Beat until smooth and fluffy, another 1-2 minutes, scraping down the sides as needed.
1 large egg + 1 egg yolk, 1 teaspoon vanilla extract
Finish the dough (2 min): With the mixer on low, slowly add the dry ingredients and mix until just combined. Fold in the mini chocolate chips with a rubber spatula.
1 cup mini chocolate chips
Scoop and roll (15 min): Use 1 teaspoon to scoop out the dough and roll into balls. Place on the baking sheet about 1–2 inches apart. Press a few extra mini chocolate chips on top.
Bake (5–7 minutes): Bake until the edges are set and the centers still look slightly soft. Finish with flaky salt if desired or press a few more mini chips into the tops. Let them cool completely on the sheet pan.