Set the oven to 325°F (165°C). Line two baking sheets with parchment paper or silicone baking mats.
In a large mixing bowl or stand mixer, beat the softened butter on medium speed for about 1 minute, until smooth and creamy. Add the brown sugar and beat for 2 to 3 minutes, until light and fluffy.
¾ cup unsalted butter, 1½ cups brown sugar
Mix in the egg, egg yolk, and vanilla extract until fully combined. Add the all-purpose flour, baking soda, baking powder, and salt. Mix gently until partly combined—do not overmix.
1 large egg, 1 large egg yolk, 2 teaspoons vanilla extract, 1 cup all-purpose flour, ½ teaspoon baking soda, ¾ teaspoon baking powder, ¾ teaspoon salt
Stir in the rolled oats, then add the oat flour just until incorporated. Use a spatula to evenly mix in the semisweet chocolate chips.
1 cup rolled oats, 1¼ cups oat flour, 2½ cups semisweet chocolate chips
Scoop about 1¾ tablespoons of dough (roughly 2-inch-wide balls) and place them on the prepared baking sheets, spacing each about 2 inches apart.
Bake for 9 to 10 minutes, until the edges are golden and set while the centers remain slightly shiny.
Right after baking, use two spoons to gently push the edges of each cookie toward the center to make them thicker and perfectly round. Let the cookies rest on the pan for 5 to 10 minutes before transferring them to a wire rack to cool completely.
While still warm, press extra chocolate chips on top and sprinkle with flaky salt for a bakery-style finish.