Preheat oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving an overhang on the long sides for easy lifting. Lightly grease the parchment.

In a large bowl, whisk together the melted butter, granulated sugar, and brown sugar until glossy and thick, about 1–2 minutes. Whisk in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract and instant espresso powder (if using) until smooth.

Sift the flour, cocoa powder, salt, and baking powder into the bowl with the wet ingredients. Gently fold with a spatula just until no dry streaks remain, being careful not to overmix.

Pour the thick brownie batter into the prepared pan and spread it evenly into the corners with a spatula or the back of a spoon.

Bake for 22–26 minutes, or until the edges are set and a toothpick inserted in the center comes out with moist crumbs but not wet batter. The center may look slightly soft; it will continue to set as it cools.

While the brownies bake, combine the chocolate chips, 2 tablespoons butter, and heavy cream in a small saucepan over low heat. Stir constantly until melted and smooth, then remove from heat and stir in a pinch of salt.

As soon as the brownie base comes out of the oven, sprinkle the mini marshmallows and chopped nuts evenly over the hot surface so they begin to soften.

Drizzle the warm chocolate mixture over the marshmallow and nut layer. Gently spread if desired, or leave it rustic so some marshmallows show through.

Return the pan to the oven for 3–5 minutes, just until the marshmallows puff and take on a light golden color in spots. Watch closely so they don’t burn.

Cool the brownies in the pan on a wire rack for at least 1–2 hours. For cleaner cuts, chill in the refrigerator for 45–60 minutes once no longer hot. Lift out using the parchment overhang and cut into 18–24 squares with a sharp knife, wiping the blade between cuts. Garnish with extra mini marshmallows and flaky sea salt if desired.