Prep: Preheat the oven to 350°F. Line several light-colored baking sheets with parchment paper or silicone baking mats.
Melt the chocolate: In a medium heatproof bowl, combine the chopped chocolate, butter, and oil. Microwave in 20-second intervals, stirring between each, until smooth and fully melted.
8 oz semisweet chocolate bars, 4 Tbsp. unsalted butter, 1/4 cup neutral oil
Add cocoa powder: Whisk in the cocoa powder until incorporated. Set aside to cool for about 5 minutes Start Timer. The mixture should still be warm but not hot.
3 Tbsp. Dutch-process cocoa powder
Whip the eggs and sugar: In a large mixing bowl, combine the eggs, egg yolk, granulated sugar, brown sugar, and vanilla extract. Using a hand mixer or stand mixer fitted with the whisk attachment, beat on medium-high speed for 5-7 minutes Start Timer, or until the mixture becomes pale, thick, and ribbony. This step is key for getting the crinkly tops.
2 large eggs, 1 large egg yolk, 1 cup + 2 Tbsp granulated sugar, 2 Tbsp. light brown sugar, 2 tsp. vanilla extract
Combine: With the mixer on low speed, slowly stream in the warm chocolate mixture. Mix until fully incorporated.
Add dry ingredients: Add the flour, baking powder, and salt. Mix on low speed just until no dry streaks remain. Do not overmix. (You can also add in additional chopped chocolate here if desired.
2/3 cup + 1 Tbsp. all-purpose flour, 1/2 tsp. baking powder, 3/4 tsp. Diamond Crystal kosher salt, 1/2-1 cup chopped chocolate
Scoop: Using a 2 Tbsp cookie scoop, portion the batter onto the prepared baking sheets, leaving about 2 inches between each cookie. Note: The cookie dough will look almost like brownie batter right when you bring it together. This is totally fine! The longer it sits, the more the dough will start to behave like cookie dough. Each batch of cookies you bake might have a slightly different crinkled appearance on top due to the temperature of the dough.
Bake: Bake for 9-10 minutes Start Timer, or until the tops are crackly and the edges look set while the centers still appear slightly soft. Do not overbake.
Cool: Allow the cookies to cool on the baking sheet for 5 minutes Start Timer before transferring to a wire rack. Sprinkle with flaky sea salt, if desired.
Flaky sea salt