Prep the pan: Preheat your oven to 350°F and line an 8x8-inch baking pan with parchment paper.
Make the shortbread dough: Stir together the melted butter and vanilla, then pour the mixture over the separately combined dry ingredients and mix until a soft, crumbly dough forms. Press about two-thirds of the dough into the prepared pan and reserve the remaining dough for the crumble topping.
240 g all-purpose flour, 200 g granulated sugar, 170 g unsalted butter, ½ teaspoon salt, 1 teaspoon vanilla extract
Par-bake the crust: Bake for 12–15 minutes until lightly golden around the edges and dry on top. Let cool for about 5 minutes while you prepare the brownie filling.
Make the brownie filling: Melt the chopped chocolate and butter together until smooth. Let cool slightly, then whisk in the sugar, eggs one at a time, and vanilla until combined. Fold in the flour, cocoa powder, salt, and espresso powder until just incorporated.
170 g dark chocolate, 85 g unsalted butter, 150 g granulated sugar, 2 large eggs, 1 teaspoon vanilla extract, 30 g all-purpose flour, 15 g cocoa powder, ¼ teaspoon salt, 1 teaspoon espresso powder
Assemble the bars: Pour the brownie filling over the crust and spread evenly. Crumble the reserved dough over the top in uneven chunks and finish with a sprinkle of flaky salt.
Flaky salt
Bake and cool: Bake for 25–30 minutes until the crumble topping is golden and the brownie layer is set around the edges with a slight jiggle in the center. The internal temperature should be 180F.Let cool completely before slicing, about 2 hours. For extra clean cuts, chill the bars for 30–60 minutes before serving.