I adapted a Classic Brownies recipe from Cook’s Illustrated, which was reprinted by Toast Point in 2007.

Makes one 9 x 13-inch baking dish/pan of brownies
Contains: eggs, milk, tree nuts, wheat

Brownie batter:
1 1/4 cups all-purpose flour
3/4 teaspoon baking powder
1/2 teaspoon salt
200 grams (6 oz) sweetened baking chocolate
170 grams (12 tablespoons) unsalted butter
1 1/4 cups sugar
4 eggs
1 tablespoon vanilla extract (or 1 packet of vanilla sugar, 7 grams))

Chestnut mixture:
250 grams pureed chestnuts**
3 tablespoons cream (light or heavy cream)

*If you are using unsweetened baking chocolate, then increase the total amount of sugar to 2 1/4 cups. Also, I prefer using a sweetened milk chocolate, but if you are using a lightly sweetened dark chocolate, you may want to increase the sugar as well to 1 1/2 or 2 cups, depending on your preferences.
**I use a frozen chestnut puree that has a little sugar added to it.

Instructions:

1. Optional: line the baking pan with aluminum foil, which makes it easier to lift the brownies out. Otherwise, just grease the pan with some butter.

2. Whisk together the dry ingredients for the brownie batter in a bowl: flour, salt and baking powder. Set aside.

3. Prepare the chestnut mixture, stirring the cream into the chestnut paste. Set aside.

4. In a heat-proof bowl, melt the chocolate and butter together until the mixture is smooth, over a pot of simmering water. Gradually whisk in the sugar. Next, whisk in the eggs, one at a time, until thoroughly combined (make sure the mixture isn’t too hot and that you whisk quickly so the eggs don’t cook). Finally, whisk in the vanilla.

5. In three additions, gently fold the flour mixture into the chocolate mixture just until well-combined.

6. Into the prepared baking dish, pour in about half of the brownie mixture. Then, add small and evenly dispersed spoonfuls of the chestnut mixture on top of the brownie mixture. Next, gently drag a fork or skewer through the chestnut mixture to create a swirled or marbled appearance.

7. Pour the remaining brownie batter on top of the swirled chestnut mixture, gently spreading it until the chestnut is completely covered.

8. Bake at 160°C / 325°F for 40-45 minutes, until a skewer inserted into the center of the brownies comes out with a few moist crumbs attached.

Please note: The original recipe says to let the brownies cool for about 2 hours on a wire rack before lifting them neatly out of the pan with the foil. I usually can’t wait that long and eat the brownies while they’re still warm.