1 1/2sticks (170 grams) unsalted butter, at cool room temperature
1 1/4cups(250 grams) packed dark brown sugar
3/4cup(150 grams) granulated sugar
2large eggs plus one egg yolk,at cool room temperature
1teaspoonvanilla extract
3cups(297 grams) old-fashioned rolled oats
1 1/2cups(255 grams) semisweet chocolate chips
Instructions
Preheat the oven to 350°F. Line large baking sheets with parchment paper.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon.
In a large bowl, use an electric mixer to beat the butter, brown sugar, and granulated sugar on medium-high speed until smooth and well combined, about 1 to 2 minutes. Beat in the eggs and vanilla. On low speed, gradually add the flour mixture and beat until just combined. Stir in the oats and chocolate chips.
Using a large spring-loaded scoop, drop 3-tablespoon sized balls of dough onto prepared baking sheets. Flatten slightly with the bottom of a measuring cup.
Bake for about 14 to 15 minutes, or until the edges are slightly browned, rotating baking sheets halfway through. If you prefer a slightly softer cookie, bake about 12-13 minutes, or until the edges are slightly brown but the middle still looks underdone (will firm up while cooling). Let the cookies cool on baking sheets for 5 minutes before removing to wire racks to cool completely.