These Chocolate Chubbies from Sarabeth's Bakery fall somewhere between a brownie and a cookie. Unlike other brownie cookies, these don't spread out thin as they bake, but rather retain their "chubby" mounded shapes because they are packed full of chopped nuts and chocolate chips.
Position racks in the center and top third of the oven and preheat to 350°F. Line 2 half-sheet pans with parchment paper.
Bring 1 inch of water to a simmer in a medium saucepan over low heat. Put the butter in a wide, heatproof bowl, and melt the butter over the hot water in the saucepan. Add the semisweet and unsweetened chocolate, stirring often, until melted and the mixture is smooth. Remove the bowl from the heat and let stand, stirring occasionally, until cooled slightly but still warm, about 5 minutes.
Sift the flour, baking powder, and salt together into a medium bowl. Whip the eggs in the bowl of a stand mixer fitted with the whisk attachment on medium-high speed until the eggs are foamy and lightly thickened, about 30 seconds. Increase the speed to high and gradually add the sugar, then the vanilla. Whip until the eggs are thick and pale yellow, about 3 minutes.
Reduce the mixer speed to medium and beat in the tepid chocolate, making sure it is completely incorporated. Change to the paddle attachment and reduce the mixer speed to low. Gradually add the flour mixture. Remove the bowl from the mixer. Using a rubber spatula, stir in the chocolate chips, pecans, and walnuts, making sure the chunky ingredients are evenly distributed at the bottom of the bowl.
Using a 2-inch ice-cream scoop (#24 size), portion the batter onto the prepared pans, placing the cookies about 1 ½ inches apart. Bake the cookies immediately—if you wait, they won’t be shiny after baking. Bake, switching the position of the pans from top to bottom and front to back about halfway through baking, until the cookies are set around the edges (if you lift a cookie from the pan, the edges should release easily, even if the center of the cookie seems underdone), 17 to 20 minutes. Do not overbake. Cool completely on the pans.
Notes
The cookies can be stored in an airtight container at room temperature, with the layers separated by parchment paper, for up to 3 days.
You should be able to fit 16 chocolate chubbies per pan since they don’t spread out much. Otherwise, if needed a third parchment paper-lined pan can bake after the first batch is complete (it's ok to leave it out at room temp briefly while you wait). Bake the lone pan in the center of the oven for a couple minutes less than what it took to bake the 2 pans together.
If you have any cracks, crevices or pockets in the dough after scooping, they will stay that way. These brownie cookies don’t really spread out and will mostly retain the shape they have once scooped. Feel free to carefully smooth out the unbaked scoops if you'd like to fill in any of the imperfections before baking.
If you don't have a #24 scoop (2-inches diameter and about 1 ⅓ ounce capacity) you could use a slightly smaller scoop and yield more chocolate chubbies. Just note that you may need to reduce the baking time slightly.