For the Chocolate Cake
- 1 cup plus 2 tablespoons all-purpose flour
- 1 cup granulated sugar
- 1/ 4 cup plus 2 tablespoons unsweetened Dutch-process cocoa powder
- 3/ 4 teaspoon baking powder
- 1 teaspoon baking soda
- 1/ 2 teaspoon kosher salt
- 1/ 2 cup sour cream
- 1/ 4 cup canola oil, or grapeseed oil
- 1 large egg
- 1 teaspoon vanilla extract
- 1/ 2 cup freshly brewed hot espresso or strong coffee
To Make the Chocolate Cake
Preheat oven to 350ºF. Line an 8x8-inch square baking pan.
In the bowl of a stand-mixer, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt. In a separate bowl, whisk together sour cream, oil, egg, and vanilla extract.
Using the paddle attachment, on low speed, add the wet ingredients to the bowl and mix just until combined. Stop and scrape down the sides and bottom of the bowl. On low, slowly stream in the hot espresso/coffee and mix to combine. Pour batter into prepared baking pan.
Bake for 30 – 35 minutes, until a toothpick inserted into the center of the cake comes out clean or with minimal crumbs. Remove from oven and set on cooling rack for 15 – 20 minutes. Using the parchment paper sides, gently pull the cake from its pan and set back on cooling rack to finish cooling. Chocolate cake can be plastic wrapped securely and stored in the freezer for up to 1 month if desired.
Once the cake has cooled completely, dollop on the dark chocolate frosting in thick mounds around the top of the cake. Using an off-set spatula, or a spoon, lightly begin moving the frosting towards the outer edges of the cake in small motions; leaving about a 1/4-inch border around the edge. Dollop on more frosting as needed. (Tip: Try to avoid using a heavy hand here – just lightly press the surface of the frosting around so that it remains in a thick layer.) Make light swooping motions with the spatula to create nice indentions into the frosting. Sprinkle generously with rainbow sprinkles, nonpareils, and flaky sea salt. Cut into nine squares (or however small or large you want) and serve. Store leftover cake, in an airtight container, at room temperature for up to five days.
For the Dark Chocolate Frosting
- 8 ounces 60-70% dark chocolate
- 1/ 4 cup unsweetened Dutch-process cocoa powder
- 1/ 4 cup boiling water
- 1 cup unsalted butter, at room temperature
- 1/ 4 cup plus 3 tablespoons powdered sugar, sifted
- 1/ 8 teaspoon fine sea sakt, or kosher salt
- 1 teaspoon vanilla extract
- Flaked salt, optional
To Make the Dark Chocolate Frosting
In a heat-proof bowl, gently melt the dark chocolate over a double boiler. Or, place the bowl of chocolate in the microwave and melt in 30 second intervals, stirring between each heating, just until melted. Set the melted chocolate bowl aside until cool to the touch.
In a small bowl, combine the cocoa powder and hot water until smooth. Set aside until cool.
In the bowl of a stand-mixer, fitted with the paddle attachment, cream the butter until fluffy, about 1 minute. Add the powdered sugar and salt. Mix, on low, until combined and then increase speed to medium-high and mix until fluffy, about 2 – 3 minutes. Add vanilla and mix until combined. Stop and scrape down the sides and bottom of the bowl as needed.
On low, add in melted chocolate and mix until combined. Scrape down bowl and add in cocoa mixture. Mix until thoroughly combined. The frosting will be soft, but if you prefer it firmer simply place it in the refrigerator to firm up (stirring every few minutes until it reaches the desired consistency). Store any leftover chocolate frosting, in an airtight container, in the freezer for up to one month.