Preheat the oven to 350°F.

Grease an 8 x 8-inch square baking pan.

Line the pan with parchment paper, allowing the paper to slightly overhang the sides.

Set the prepared pan aside.

In a mixing bowl, whisk together the all-purpose flour, espresso powder, baking powder, and salt.

Set the dry mixture aside.

Place the cubed unsalted butter and granulated sugar in a medium saucepan.

Heat the mixture over medium-low heat, whisking frequently, until the sugar is dissolved and the mixture is smooth and shiny.

Do not allow the butter mixture to bubble or foam.

If using a candy thermometer, heat the mixture until it reaches approximately 50°C.

While the butter mixture is heating, place the eggs, Dutch-process cocoa powder, and vanilla bean extract in the bowl of a stand mixer fitted with the whisk attachment.

Whisk on medium speed for approximately 2 minutes, or until the mixture is glossy, thick, and smooth.

With the mixer running on medium speed, slowly stream the warm butter and sugar mixture into the egg and cocoa mixture.

Continue whisking until the batter is evenly combined.

Stop the mixer and sift the dry ingredients over the chocolate mixture.

Whisk just until combined.

Do not overmix the batter.

Remove the bowl from the stand mixer.

Fold in the dark chocolate chips using a rubber spatula or wooden spoon until evenly distributed.

Pour the brownie batter into the prepared baking pan.

Smooth and level the top with an offset spatula.

Bake for 25 minutes, or until the edges are firm and the center is just set.

Insert a wooden skewer into the center of the brownies to check for doneness. The skewer should come out with a few moist crumbs, not completely clean.

Remove the pan from the oven.

Allow the brownies to cool completely in the pan before frosting.

Place the finely chopped dark chocolate in a medium heatproof bowl.

Set the bowl over a saucepan filled with a few inches of barely simmering water.

Ensure that the bottom of the bowl does not touch the water.

Heat the chocolate over medium-low heat, stirring frequently, until melted and smooth.

Remove the melted chocolate from the heat.

Set it aside to cool slightly.

In the bowl of a stand mixer fitted with the beater attachment, combine the room-temperature butter and confectioners’ sugar.

Beat on medium speed for approximately 4 minutes, or until pale and fluffy.

Reduce the mixer speed to medium-low.

Slowly stream in the cooled melted chocolate.

Increase the mixer speed to medium.

Beat for an additional 2 minutes, or until fully combined.

Add the heavy cream, Dutch-process cocoa powder, espresso powder, and vanilla bean extract.

Beat until the ingredients are incorporated.

Increase the mixer speed to medium-high.

Beat for approximately 1 minute, or until the frosting is smooth, aerated, and slightly lightened in color.

Once the brownies are completely cool, spread the frosting evenly over the top.

Use an offset palette knife or the back of a metal spoon to create a smooth or swirled finish.

Sprinkle the frosted brownies lightly with flaked salt or fleur de sel.

For cleaner slicing, chill the brownies for approximately 30 minutes before cutting.

Slice into 16 squares.

Serve chilled, at room temperature, or slightly softened after refrigeration.