1 tablespoon unsweetened cocoa powder, or more to taste
7 (1.4-ounce) Heath bars or other chocolate-covered toffee candy bars
2 cups sifted all-purpose flour
1 1/4 teaspoons baking soda
1/4 teaspoon salt
8 tablespoons (1 stick) unsalted butter, softened
1 cup packed light brown sugar
1/2 cup granulated sugar
1 teaspoon pure vanilla extract
2 large eggs, lightly beaten, at room temperature
1 1/2 cups buttermilk, at room temperature
1/2 cup chopped pecans or walnuts
Preheat oven to 325 F, placing one of the racks in the lower third of the oven. Grease a 9x13-inch baking pan or spray with oil-and-flour baking spray. Sprinkle with the cocoa, dumping out any that doesn’t adhere to the pan.
Chop the candy bars coarsely. You want pieces about twice as chunky as chocolate chips, and more irregular. Scoop the candy bar chunks into a shallow bowl and stash in the freezer until you need them.
Whisk the flour, baking soda and salt together in a small bowl. Set aside.
With an electric mixer at high speed, cream the butter, brown sugar and granulated sugar together until light and fluffy, about 5 minutes. Reduce the mixer speed to medium, and beat in the vanilla and then the eggs, drizzling the eggs in steadily. Mix in about a third of the flour mixture at a time, alternating with a third of the buttermilk at a time, stopping as needed to scrape down the sides of the bowl. Mix after each addition just until combined. Stir in the pecans and half of the cold candy bar chunks. The batter will be thick. Spoon the batter into the prepared pan and smooth the top. Scatter the remaining candy bar chunks over the batter.
Bake for 40 to 45 minutes, until a toothpick inserted in the center comes out clean. Cool in the pan at least 45 minutes before cutting. Sprinkle the cake either before or after slicing with additional cocoa, if you wish. Makes 12 servings.