Preheat the oven to 350°F (175°C) and grease a 9 x 13-inch baking pan.
In a mixing bowl, cream together the brown sugar and room-temperature butter until light and fluffy.
Gradually add the flour to the creamed mixture and mix until just blended. Press the dough evenly into the bottom of the prepared pan to form the crust.
Gently melt the chocolate and additional butter in a microwave-safe bowl in short bursts, stirring between intervals until smooth; stir in espresso if using.
Allow the melted chocolate mixture to cool slightly, then stir in the granulated sugar and vanilla extract.
Add the eggs one at a time, mixing until each is incorporated, then stir in the flour (if additional is called for) and a pinch of salt until just combined.
Pour the chocolate filling over the prepared crust and spread into an even layer.
Bake on the middle rack for about 25–27 minutes or until set; remove from the oven and immediately place the pan in the refrigerator to chill.
To make the glaze, gently heat chocolate, cream, and butter together (microwave in short bursts or use a double boiler) until the chocolate and butter are melted, then stir until smooth.
Cool the glaze for about 10 minutes, then spread it evenly over the chilled brownies and return the pan to the refrigerator to set.
When the brownies are fully chilled and the glaze is set, cut into bars and serve.