Triple-Chocolate Mousse Cake | America's Test Kitchen

Ingredients

• 6 tablespoons (3/4 stick) unsalted butter , cut into 6 pieces, plus extra for greasing pan
• 7 ounces (198 grams) bittersweet chocolate , chopped fine (see note)
• 3/4 teaspoon instant espresso powder
• 1 1/2 teaspoons vanilla extract
• 4 large eggs , separated
• pinch table salt
• 1/3 cup packed (about 2 1/2 ounces/71 grams) light brown sugar , crumbled with fingers to remove lumps
• 2 tablespoons cocoa powder , preferably Dutch-processed
• 5 tablespoons hot water
• 1 1/2 cups cold heavy cream
• 1 tablespoon granulated sugar
• 1/8 teaspoon table salt
• 3/4 teaspoon powdered gelatin
• 1 tablespoon water
• 6 ounces (170 grams) white chocolate chips (see note)
• Shaved chocolate or cocoa powder for serving, optional (see note)