Yield: 1 generously frosted 8×8-inch cake, which we cut into 16 small squares.
Could also yield: 1 generously frosted 8-inch or 9-inch round cake, 12 very generously frosted cupcakes, or it could be doubled and baked in a 9×13-inch pan. Could also be scaled and stacked as a birthday cake.
Cake
6 tablespoons (85 grams) unsalted butter, at room temperature
3/4 cup (145 grams) firmly packed dark brown sugar
2 tablespoons (25 grams) granulated sugar
1 large egg
1 large egg yolk
3/4 cup (175 ml) buttermilk (see Notes for other options)
1 teaspoon (5 ml) vanilla extract
1/2 cup (41 grams) Dutch cocoa powder
1 cup (125 grams) all-purpose flour
1/4 teaspoon baking soda
1/2 teaspoon baking powder
1/2 teaspoon table or fine sea salt
Frosting
2 ounces (55 grams) unsweetened chocolate, melted and cooled
1 1/2 cups (180 grams) powdered sugar (sifted if lumpy)
1/2 cup (4 ounces or 115 grams) unsalted butter, at room temperature
Pinch of fine sea salt (optional)
1 tablespoons cream or whole milk
1/2 teaspoon vanilla extract
Make the cake:
Heat oven to 350°F (175°c). Line the bottom of an 8-inch square cake pan with parchment paper, and either butter the parchment and exposed sides of the pan, or spray the interior with a nonstick spray. In a large bowl, use a hand or stand mixer to beat the butter and sugars until fluffy; scrape down bowl. Add the egg, yolk and vanilla and beat until combined, then the buttermilk and mix again. Scrape the bowl down well and don’t worry if the batter looks uneven. Place your flour, cocoa, baking soda, baking powder and salt in a sifter and shake it over the batter bowl. Stir on low until just combined; scrape down bowl a final time. Scoop batter into prepared pan and smooth flat. Bake for 25 to 35 minutes, until a toothpick inserted into the center comes out clean. Let cool for 5 to 10 minutes in cake pan on cooling rack, then flip out onto rack or serving plate to completely finish cooling before frosting.
Make the frosting:
Place frosting ingredients in a food processor and run machine to to mix. Scrape down bowl then process until smooth and somewhat fluffed. [Don’t have a food processor? Beat butter, powdered sugar and salt, if using, in a large bowl with an electric mixer until fluffy. Pour in chocolate, milk and vanilla, beat until combined, then one more minute to whip it further.] Scoop the frosting onto the cooled chocolate cake and swirl it around. Finish with rainbow sprinkles. Cut into squares.
Cake keeps at room temperature for a day or two, or in the fridge up to a week.
Note:
Red wine chocolate cake, mocha chocolate cake: You could replace the buttermilk in full with yogurt or sour cream thinned with a little milk or water, with red wine or replace 1/2 cup of it with strong coffee. (Keep 1/4 cup buttermilk so you’ll have an acidic ingredient to wake up the baking soda.)