Preheat your oven to 350°F. Grease an 8x8-inch metal baking pan with butter, then line it with a piece of parchment paper, leaving overhang on two opposite sides for easy removal. Lightly butter the parchment as well to ensure nothing sticks.
In a medium saucepan over low heat, melt the butter and chopped bittersweet chocolate, stirring occasionally. Once most of the chocolate has melted, remove from heat, stir in the espresso powder, and set aside to cool slightly.
½ cup unsalted butter, ¾ cup chopped bittersweet chocolate, 1 teaspoon instant espresso powder
In a large mixing bowl, beat the granulated sugar, brown sugar, eggs, vanilla, and salt using a hand mixer on medium speed for about 2 minutes, until the mixture becomes slightly thick and glossy. To keep your bowl from sliding around, place a damp kitchen towel underneath. Reduce the speed to low and slowly pour in the melted chocolate mixture along the side of the bowl (rather than directly over the beaters) to prevent splattering. Mix until fully incorporated and no streaks remain.
¾ cup granulated sugar, ½ packed cup dark brown sugar, 2 large eggs, 1 teaspoon vanilla extract or paste, 1 teaspoon kosher salt
Add the flour and cocoa powder, then mix on low speed just until there are no visible dry spots. Gently fold in about ¾ of the chopped milk and white chocolate, reserving the rest for topping. Pour the batter into the prepared pan and smooth it into an even layer. Scatter the remaining chocolate pieces over the surface.
½ cup all-purpose flour, ½ cup Dutch-processed cocoa powder, ¾ cup chopped milk chocolate, ¾ cup chopped white chocolate
Bake for 15 minutes, or until the top just starts to set. Carefully remove the pan from the oven and firmly tap it on the counter once or twice (placing a kitchen towel underneath if needed). This helps create cracks on the surface, giving the brownies their signature crinkle and ensuring a fudgy interior without being too dense.
Sprinkle with flaky sea salt, then return the pan to the oven and bake for another 12-14 minutes, or until the edges are set and the center no longer jiggles when gently shaken.
Flaky sea salt
Let the brownies cool completely in the pan at room temperature for about 2 ½ hours. Once fully set, lift them out using the parchment overhang and slice into 16 squares. Store leftovers in an airtight container at room temperature for up to 4 days.