Shortbread Crust:
Preheat the oven to 350°F (175°C). Line an 8×8 inch square baking dish with parchment paper, allowing the paper to overhang the sides for easy removal later.
In a bowl, mix the flour, sugar, salt, and vanilla. Add the cold, diced butter and use a pastry cutter (or food processor) to cut the butter into the dry ingredients until the mixture forms moist crumbs.
Press the crumb mixture evenly into the bottom of the prepared pan. Bake for 15 minutes until pale golden brown. Set aside to cool slightly.
Reduce oven temperature to 325°F (160°C).
Brownies:
In a large saucepan, melt the butter and sugar over medium heat until it begins to bubble. Cook for 2 minutes, then remove from heat.
Stir in the chocolate chips and vanilla until the chocolate is melted and the mixture is smooth but grainy. Let cool for 10 minutes.
Beat in the eggs, one at a time, until smooth.
In a separate bowl, sift together the cocoa powder, flour, salt, and espresso powder (if using). Stir the dry ingredients into the batter until just combined. The batter will be thick.
Spread the brownie batter evenly over the baked shortbread crust.
Bake at 325°F for exactly 25 minutes. The top should be crackled and slightly puffed. Let the brownies cool completely.
Ganache:
Finely chop the dark chocolate and place it in a bowl.
Heat the cream in a small saucepan over medium heat until it begins to simmer around the edges. Remove from heat and pour the hot cream over the chopped chocolate.
Stir until the ganache is smooth and shiny.
Spread the ganache over the cooled brownies and let it set for about an hour. If desired, sprinkle with reserved chopped chocolate for a decorative finish.
Cutting:
Once the ganache is set, lift the brownies out of the pan using the parchment paper overhang. Place them on a cutting board and use a sharp knife to cut into clean slices, wiping the knife between cuts for neat edges.