Step 1

Heat 1 tablespoon vegetable oil in a large, heavy-bottomed pot with a lid over medium-high heat until shimmering. Add 1 teaspoon of the popcorn kernels to test if the oil is hot enough. Once a few kernels start popping, add the remaining popcorn kernels. (Be careful, the popping kernels can fly out of the pot!) Cover the pot and cook, shaking the pot occasionally, until the popcorn stops popping and there is about 5 seconds of silence between each pop, 2 1/3 to 3 minutes total.

Step 2

Pour the popcorn into a large bowl, taking care not to include any unpopped kernels. (Makes about 5 1/2 cups.)

Step 3

Carefully wipe out the inside of the pot with a paper towel to remove any stuck pieces of popcorn. Place back onto the stovetop over low heat. Add 2 sticks (1 cup) unsalted butter and cook, stirring occasionally, until fully melted but not browned, 2 to 3 minutes. Immediately pour into a large, heat-proof bowl and let cool on the counter for 30 minutes.

Step 4

Meanwhile, place 2 1/4 cups all-purpose flour, 3/4 teaspoon kosher salt, 1/2 teaspoon baking powder, and 1/2 teaspoon baking soda in a medium bowl and whisk until combined. Arrange 2 racks to divide the oven into thirds and heat the oven to 350°F. Line 3 baking sheets (or as many as you have) with parchment paper.

Step 5

Add 1 cup granulated sugar, 3/4 cup packed light brown sugar, 1 large egg, 1 large egg yolk, and 1 tablespoon vanilla extract to the bowl of melted butter. Whisk until completely smooth and glossy. Add the flour mixture and stir with a rubber spatula or wooden spoon until the flour is completely incorporated and a smooth dough forms.

Step 6

Smash the popcorn with your hands to break it up into small, irregular pieces. You want the popcorn to be slightly larger than the size of peas with random, whole pieces still intact. Discard any pieces of hard kernels that may still be stuck to the popcorn. Add to the dough and gently fold it in.

Step 7

Scoop out the dough with a 1 1/2-inch wide cookie scoop or into 32 (about 2 rounded tablespoon) portions, and roll into balls. Place the dough balls at least 2 inches apart on the prepared baking sheets, 10 to 12 per sheet. Sprinkle the tops with flaky salt, if using.

Step 8

Bake 2 of the sheets, rotating the baking sheets from back to front and top to bottom halfway through, until cracks start forming, 11 to 13 minutes total. Err on the side of underbaking them (these cookies will not brown much). Let cool on the baking sheets for 5 minutes, then transfer onto a wire rack to cool completely.

Step 9

Bake the last sheet of cookies on either rack, rotating the pan front to back halfway through baking. (If reusing one of the baking sheets, let it cool for 15 minutes before baking the last batch on it.)