For the Cake:

1/4 cup bittersweet chocolate chips

1 tablespoon espresso powder

1/3 cup boiling water

1/4 cup canola oil

1/4 cup full-fat Greek yogurt (or sour cream)

1/4 cup whole milk

1 large egg plus 1 egg yolk, room temperature

1 teaspoon vanilla extract

1 cup plus 2 tablespoons (140 g) all-purpose flour

3/4 cup (150 g) granulated sugar

1/2 cup (50 g) Dutch process cocoa powder

1 teaspoon baking soda

1/2 teaspoon baking powder

1/2 teaspoon salt

For the Chocolate Frosting:

1/2 cup bittersweet chocolate (chopped or chips)

1 tablespoon Dutch process cocoa powder

1/2 cup (113 g) unsalted butter, room temperature

1 teaspoon vanilla extract

Pinch of salt

2 cups (240 g) powdered sugar, sifted

1–2 tablespoons whole milk or heavy cream

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Preheat oven to 350°F (175°C). Grease or line an 8×8-inch square baking pan with parchment paper.
In a heatproof bowl, combine chocolate chips and espresso powder. Pour boiling water over them and let sit for 1 minute. Stir until smooth and melted.
Whisk in canola oil, Greek yogurt, milk, eggs, and vanilla until well combined and glossy.
In a large bowl, whisk together flour, sugar, cocoa powder, baking soda, baking powder, and salt.
Pour the wet ingredients into the dry ingredients and stir gently until just combined.
Spread the batter evenly into the prepared pan and smooth the top.
Bake for 28–32 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Let cool completely in the pan.
For the frosting: Melt the bittersweet chocolate (over a double boiler or in the microwave in short bursts). Let cool slightly.
In a bowl, beat butter until creamy, then mix in cooled melted chocolate, cocoa powder, vanilla, and salt until smooth.
Gradually add powdered sugar and beat until combined. Add milk or cream 1 tablespoon at a time until frosting is silky and spreadable.
Spread frosting evenly over the cooled cake. Optionally, top with chocolate shavings or a sprinkle of flaky sea salt.

Use Dutch process cocoa for the richest flavor and color.

Espresso powder enhances chocolate taste without adding coffee flavor.

Don’t overmix the batter to maintain a tender crumb.

Check doneness early to avoid overbaking.

Store covered at room temperature for 2 days or refrigerate for up to 5 days.