From Baking: From My Home to Yours, by Dorie Greenspan
1 1/4 cups plus 2 tablespoons sugar
1 tablespoon plus 2 teaspoons ground cinnamon
1 1/2 teaspoons instant espresso powder
1 3/4 cups all-purpose flour
2 teaspoons baking powder
pinch of salt
3/4 cup whole milk
2 large eggs
1/2 teaspoons pure vanilla extract
10 tablespoons unsalted butter, melted and cooled
3 ounces bittersweet chocolate, finely chopped, or 1/2 cup mini chocolate chips
6 ounces bittersweet chocolate, finely chopped
2 1/2 tablespoons unsalted butter, cut into 4 pieces
Getting Ready: Center a rack in the oven and preheat the oven to 350 F. Butter an 8-inch square baking pan and line the bottom with parchment or wax paper. Place the pan on a baking sheet.
To Make the Cake: Stir 2 tablespoons of the sugar, 2 1/2 teaspoons of the cinnamon and the espresso together in a small bowl.
In a large bowl, whisk together the flour, the remaining 1 1/4 cups sugar, the baking powder, salt and the remaining 1 tablespoon cinnamon. In another bowl, whisk together the milk, eggs and vanilla. Pour the liquid ingredients over the flour mixture and gently whisk until you have a homogenous batter. Now, using the whisk or a rubber spatula, fold in the butter with a light touch, just until the butter is absorbed. You'll have a smooth, shiny batter.
Scrape half of the batter into the pan and smooth the top. Sprinkle the chocolate over the batter and dust with the cinnamon-sugar mixture. Cover with the rest of the batter and smooth the top again.
Bake for 35 to 40 minutes, or until the cake is puffed and beginning to pull away from the sides of the pan; a thin knife inserted into the center will come out clean. Transfer the cake to a cooling rack and let it rest for 15 minutes before unmolding it onto another rack. Peel off the paper, invert it onto the first rack, and cool to room temperature right side up.
To Make the Frosting: Put the chocolate and butter in a heatproof bowl and fit the bowl over a saucepan of simmering water. Cook, stirring gently and often, just until they melt. Be careful not to overheat the mixture so much that it thins out; the chocolate should be smooth, very shiny, thick and spreadable. (If it thins, leave the frosting at room temperature for a bit, until it thickens a little.)
Using an offset metal icing spatula or a table knife, spread the frosting in generous sweeps and swirls over the top of the cake. Allow the frosting to set at room temperature, then cut the cake into 9 squares, each about 2 1/2 inches on a side.
Cappuccino Squares: If you like the cappuccino flavor combination of coffee and cinnamon, you can easily switch the balance in this recipe by adding a jolt of coffee flavor to the batter. Just mix 1 tablespoon instant espresso into the milk and warm the milk in a microwave oven until it is hot enough to dissolve the coffee. Cool the milk and carry on.
Serving: Serve the cake as is or, if you'd like, with whipped cream, creme fraiche or sour cream.
Storing: Wrapped in plastic, the cake will keep at room temperature for 2 days. It can be frozen for up to 2 months, but it's best to put the cake in the freezer unwrapped and then, when the frosting is firm, to wrap it airtight; defrost, still wrapped, overnight in the refrigerator.