1/3 cup Dutch-processed cocoa
1 1/2 teaspoons instant espresso (optional)
1/2 cup plus 2 tablespoons boiling water
2 ounces unsweetened chocolate, finely chopped
4 tablespoons (1/2 stick) unsalted butter, melted
1/2 cup plus 2 tablespoons vegetable oil
2 large eggs
2 large egg yolks
2 teaspoons vanilla extract
2 1/2 cups sugar
1 3/4 cups unbleached all-purpose flour
3/4 teaspoon table salt
6 ounces bittersweet chocolate, cut into 1/2-inch pieces
6 tablespoons butter
1 1/2 cups chopped pecans
1 cup firmly packed light brown sugar
Heat oven to 350F degrees. Make a foil sling for your 13 x 9 inch pan. Spray with nonstick cooking spray.
Whisk cocoa, espresso powder (if using), and boiling water together in large bowl until smooth. Add unsweetened chocolate and whisk until chocolate is melted. Whisk in melted butter and oil. Add eggs, yolks, and vanilla and continue to whisk until smooth and homogeneous. Whisk in sugar until fully incorporated. Add flour and salt and mix with rubber spatula until combined. Fold in bittersweet chocolate pieces, toasted walnuts and toffee bits.
Scrape batter into prepared pan. Melt the butter in a small saucepan. Add the pecans and sugar and cook, over medium heat, stirring frequently, until the sugar dissolves. Drizzle the caramel mixture over brownie batter. Bake until toothpick inserted halfway between edge and center comes out with just a few moist crumbs attached, 35 to 40 minutes. If you like more cake-like brownies, bake for 5 minutes longer. Transfer pan to wire rack and cool for an hour.
Using foil overhang, lift brownies from pan. Return brownies to wire rack and let cool completely, about 1 hour more. Cut into 2-inch squares and serve.