Adjust an oven rack to the middle of the oven. Preheat the oven to 350°F. Line two sheet pans with parchment paper.
In a small bowl, whisk together the flour and baking powder, and set aside.
In the bowl of a stand mixer fitted with the paddle, beat the eggs, sugar, and salt on medium-high speed until the mixture is pale and doubled in volume, 6 to 8 minutes. Turn the mixer to low speed and stir in the canola oil and vanilla until just combined.
While the egg mixture is beating, melt the butter and chocolate. Place the butter in a small, heavy-bottom saucepan set over low heat. Add the chocolate and melt together, stirring frequently, until smooth.
Off the heat, add the cocoa powder to the chocolate and whisk until completely combined.
Add the warm chocolate-butter mixture to the egg mixture and mix on low speed until combined. Add the flour mixture and mix on low speed until combined. Let the mixture sit at room temperature for 5 minutes.
Use a small scoop or two spoons to drop heaping tablespoons of batter onto the prepared sheet pans, spacing them at least 2 inches apart, fitting 12 on a pan.
Bake the cookies one pan at a time, rotating halfway through baking. Bake until the cookies are puffed and cracked and the edges are set, 8 to 12 minutes. Time will vary depending on the size of your cookies. I recommend checking early to be safe.
Transfer the sheet pan to a wire rack and let the cookies cool completely on the pan.
This recipe makes 30 smaller cookies, using a rounded tablespoon. If making smaller cookies, they will be done closer to 8 minutes instead of 12. If you want to make large cookies, you will get 16 cookies. The cookies will spread out a little more. The cookies are best the day of baking but will keep in an airtight container at room temperature for up to 2 days.Variation: With peanut butter — Drop 1 heaping teaspoon of peanut butter on top of each brownie dough ball and use a knife to swirl.Recipe from 100 Cookies by Sarah Kieffer