Preheat the oven to 350°F (175°C). Line a 9x9 inch (23x23 cm) square baking pan with parchment paper, leaving a 1-inch overhang on two sides for easy lifting; grease lightly with butter or nonstick cooking spray.

In a heatproof bowl set over (or gently in) simmering water, melt 6 oz chopped bittersweet/dark chocolate with 10 tablespoons unsalted butter, stirring until smooth and glossy. Remove the bowl from heat and let cool slightly for 5 minutes.

Whisk the granulated sugar and light brown sugar into the cooled chocolate-butter mixture until mostly dissolved and slightly thickened.

Whisk in the eggs one at a time, fully incorporating each before adding the next, then stir in the vanilla extract and the optional instant espresso powder for depth of flavor.

Sift together the all-purpose flour, unsweetened cocoa powder and fine sea salt into a bowl; fold the dry mixture into the wet batter gently until just combined. If using, stir in 1 tablespoon vegetable oil to help encourage a shiny crackly top.

Pour the batter into the prepared pan and smooth the top with a spatula. Bake for 28–34 minutes, until the edges are set and a toothpick inserted into the center comes out with a few moist crumbs (not raw batter). Do not overbake; the center should remain fudgy.

Remove the pan to a wire rack and allow the brownie to cool completely in the pan, at least 1 hour. Use the parchment overhang to lift the slab from the pan and transfer to a cutting board.

While the brownie cools, make the ganache: heat 3/4 cup heavy cream in a small saucepan until it begins to simmer; remove from heat and pour over 6 oz chopped dark chocolate. Let it sit 2 minutes, then whisk until smooth and shiny. Whisk in 1 tablespoon softened butter for extra gloss and set aside to thicken to a spreadable consistency at room temperature (10–20 minutes).

Trim the brownie slab edges so you have straight sides. Using a sharp serrated knife, cut the slab into rectangular blocks about 2.5 x 1.5 inches (6 x 4 cm) for house bases. For each house, cut a small triangular roof shape from the top of the block or leave as a simple peaked roof by cutting a shallow diagonal if desired. Reserve scraps for snacking.

Set the cut brownie blocks on a wire rack over a sheet pan. Spread or pour ganache over each block, using an offset spatula to smooth and fully coat the sides and top. Work quickly while ganache is pourable; if it firms too much, warm gently for a few seconds and stir.

Place coated brownie houses on a baking sheet and chill in the refrigerator for 10–15 minutes to set the ganache surface so piping won’t melt into it.

Prepare royal-style piping icing: in a mixing bowl combine confectioners' sugar with 3 tablespoons meringue powder (or the two egg whites) and 1 teaspoon lemon juice or vanilla. Add 1 tablespoon water and beat with a hand mixer on medium until the icing is glossy and holds a firm peak; add a little more water, a teaspoon at a time, if too stiff for piping. Transfer icing to a piping bag fitted with a small round tip or a zip-top bag with the corner snipped.

Pipe roof lines, window frames and decorative trim using the white icing. Press small colorful candies into the wet icing to form windows, wreaths or rooftop dots. Work systematically from larger roof outlines to small details so the base structure has time to set between additions.

Dust the assembled village lightly with powdered sugar to simulate snow. Return to the refrigerator for 10 minutes to allow piped decorations to firm fully before handling.

To serve, bring the houses to room temperature for about 10–15 minutes so the ganache softens slightly and the brownie interior is tender. Arrange on a wooden board or platter and enjoy. Store any leftovers in an airtight container in the refrigerator for up to 4 days; bring to room temperature before eating.