Place the oven rack in the center and preheat to 350°F (175°C). Lightly grease an 8-inch square metal cake pan and line its bottom with parchment, leaving an overhang if desired.

In one large bowl, whisk 1½ cups flour, 1 cup sugar, ⅓ cup natural unsweetened cocoa, 1 teaspoon baking soda, ½ teaspoon baking powder and ½ teaspoon salt until evenly mixed and free from obvious lumps.

Add one egg, ¾ cup buttermilk, ⅓ cup vegetable oil and 1 teaspoon vanilla. Whisk gently until the flour almost disappears; scrape the bowl and stop before overmixing.

Gradually whisk in ½ cup hot water in two or three additions. Stir until the batter is smooth and pourable; use caution with the hot water and do not vigorously beat.

Pour the batter into the lined 8-inch pan, level it gently and bake 30–35 minutes. Check for a set center and a toothpick showing moist crumbs but no wet raw batter, adjusting for your oven.

Cool in the pan for at least 15 minutes, then allow the cake to become completely cool before glazing, about 60 minutes in total depending on the kitchen.

Put ¾ cup semisweet chocolate chips in a heatproof bowl. Warm ¼ cup heavy cream until steaming but not boiling vigorously, pour over the chips, wait two minutes and stir until smooth.

Spread the thin chocolate glaze over the completely cooled cake and let it settle for a few minutes.

Cut into nine squares. Do not taste raw batter. Because the glaze contains heavy cream, refrigerate covered leftovers promptly at 40°F (4°C) or colder.