WHAT YOU NEED

170g unsalted butter, cut into pieces
1 ¼ cup peeled roasted chestnuts, from a bag or jar, rinsed and patted dry
2/3 cup unsweetened cocoa powder, sifted if lumpy
2 large eggs, cold from the refrigerator
1 cup granulated sugar
¼ cup packed light brown sugar
2 tsp vanilla extract
¾ cup all-purpose flour
1 tsp kosher salt/½ tsp table salt
170g semisweet chocolate (64%–70% cacao), chopped
2/3 cup heavy cream

To make

Preheat the oven and prepare the pan. Arrange an oven rack in the centre position and preheat the oven to 180°C. Line an 8 × 8-inch baking pan, preferably metal, with foil, pressing it into the corners and smoothing it up the sides. Melt the butter in a small saucepan over medium-low heat, then remove it from the heat. Use a pastry brush to coat the bottom and sides of the foil with a thin layer of the melted butter, then set the pan aside.

Cook the chestnuts. Set the saucepan of melted butter over medium heat and add the chestnuts. Bring the mixture to a simmer and sizzle the chestnuts in the butter, mashing them occasionally with a potato masher, until the chestnuts are fragrant and have broken down into a coarse paste with the biggest pieces about the size of a lentil. Remove the saucepan from the heat and add ¼ cup water and the cocoa powder and whisk until smooth. Set the saucepan aside and let cool slightly.

Whisk the eggs, sugar, and vanilla. In a medium bowl, whisk the eggs until they’re streak-free, then add the granulated sugar, brown sugar, and vanilla and whisk vigorously until the mixture is slightly pale, about 1 minute.

Make the batter. Scrape the chestnut mixture into the bowl with the egg mixture and whisk to combine, then whisk in the flour and salt. Switch to a flexible spatula and fold the mixture several times, scraping the bottom and sides of the bowl, until the batter is smooth.

To bake

Scrape the batter into the prepared pan, working it to the sides and corners with the spatula in an even layer. Bake until the surface of the brownies is puffed, dry, and firm to the touch, 22 to 28 minutes. Let the brownies cool completely in the pan.

Meanwhile, make the ganache. Place the chopped chocolate in a medium heatproof bowl. In a small saucepan, heat the cream over medium heat just until you see gentle bubbling around the sides. Remove the saucepan from the heat and set it aside for about 30 seconds to cool slightly, then pour it over the chocolate. Let the chocolate sit for about a minute, then whisk the ganache gently, starting in the centre of the bowl and working outward, until the mixture is completely smooth. Let the ganache cool, whisking occasionally, until it’s thickened and spreadable, 35 to 45 minutes.

Spread the ganache over the brownies. Use the foil to lift the cooled brownies out of the pan and transfer to a cutting board. Scrape the ganache onto the brownies and smooth across the surface with the back of a spoon or a small offset spatula. Let the brownies sit until the ganache is set, then slice with a serrated knife into a 4-by-4 grid to make 16 brownies.