Preheat the oven to 350°F. Butter and flour an 8-inch square baking pan, knocking out the excess flour. (Or line the pan with parchment, letting the parchment come up the sides and hang over- then spray the parchment. This is recommended as it's easier to get the brownies out of the pan cleanly.)
In a large saucepan, melt the bittersweet chocolate and unsalted butter with the espresso mixture over low heat, stirring until smooth, then remove the pan from heat. Cool the mixture to lukewarm and whisk in the granulated sugar and vanilla extract. Add the eggs, whisking well until the mixture is glossy and smooth. Stir in the all-purpose flour and salt until just combined and stir in walnuts (if using).
Spread the batter evenly in the pan and bake in the middle of the oven for 22 to 25 minutes, or until a tester comes out with crumbs adhering to it. Cool the brownie layer completely in the pan on a rack.
In a large bowl, use an electric mixer to combine the cream cheese and unsalted butter until light and fluffy. Add the sifted powdered sugar, vanilla extract, and ground cinnamon (if using) and beat until combined well. Spread the cream cheese layer evenly over the brownie layer. Chill the brownies for 1 hour, or until the cream cheese layer is firm.
In a large pyrex measuring cup or glass bowl, place the bittersweet chocolate, heavy whipping cream, unsalted butter, and espresso mixture. Microwave in 30 second intervals, stirring, until melted and smooth. If the glaze is steaming hot, let it cool a little bit, then pour the glaze carefully over the cream cheese layer.
Chill brownies, covered, until cold (at least 3 hours). If you used parchment, simply lift the brownies out of the pan by holding the sides of the parchment. Place on a cutting board and gently peel off the parchment. With a long, thin knife, cut the chilled brownies into 24 squares. Serve the brownies cold or at room temperature. These brownies will keep, covered and chilled in one layer, for 5 days.
