Heat oven to 350º. Butter the bottom of 8-inch square baking dish and line with parchment or foil, leaving several inches overhang on two sides. Butter the bottom and sides of the parchment or foil and set pan aside.

Melt the butter in a large bowl. Whisk in the espresso powder until it dissolves completely, then whisk in the sugar. Add the eggs, one at a time, whisking well after each addition. Stir in the vanilla. Whisk in the dry ingredients until just incorporated.

Pour the batter into the prepared pan and bake for about 30 minutes, until a toothpick inserted in the center comes out with a few moist crumbs attached. Allow to cool completely before topping with the mousse.

To make the mousse, combine the espresso and hot water in a small bowl. Whisk in Nutella and Kahlua if using. Using a hand mixer or a stand mixer fitted with the whisk attachment, beat heavy cream until it just forms stiff peaks. Using a rubber spatula, gently fold in the Nutella mixture. Scrape the mousse onto the top of the cooled brownies and spread evenly with an offset spatula. Refrigerate for at least an hour or up to overnight.

Using the parchment/foil handles, lift the brownies from the pan. Cut into squares and top as desired. Serve immediately.