How to Make It
Make the cake Preheat the oven to 350°. Grease a 9-by-13-inch baking pan with baking spray and line the bottom with parchment paper.
In a medium bowl, whisk the flour with the baking soda and salt. In a large heatproof bowl, whisk the cocoa powder with the instant coffee, then whisk in 1 1/2 cups of boiling water. Stir in the brown sugar and buttermilk until no lumps remain. Let cool for 5 minutes, then stir in the vanilla.
In a stand mixer fitted with the paddle, beat the butter at medium-high speed until lightened, about 1 minute. Beat in the granulated sugar in 3 additions, beating well after each one, until light and fluffy, about 5 minutes. Add the eggs one at a time, beating well after each addition. At low speed, beat in the dry ingredients and the cocoa mixture in 3 alternating additions, scraping down the side and bottom of the bowl as necessary, until just combined.
Scrape the batter into the prepared pan. Bake for about 35 minutes, until a toothpick inserted in the center comes out clean; rotate the pan halfway through baking. Transfer the cake to a rack and let cool for 1 hour. Run a knife around the edge of the pan, then invert the cake and remove the parchment. Let cool completely, about 30 minutes.
Meanwhile, make the frosting Put the chocolate in the bowl of a stand mixer. In a small saucepan, bring the cream and corn syrup to a simmer. Pour the hot cream over the chocolate; let stand for 2 minutes, then whisk until smooth. Let the ganache cool until barely warm, 45 minutes. In the stand mixer fitted with the whisk, beat the ganache at medium-high speed until light and spreadable, about 2 minutes.
Spread the ganache frosting over the top of the cake, top with pearls (if using) and serve.
Crispy chocolate pearls are available at specialty food shops and from amazon.com.