Moist Chocolate-Beet Cake

Ingredients

• 8 ounces (240 g) beets, unpeeled, rinsed and scrubbed free of dirt
• 7 ounces (200 g) bittersweet or semisweet chocolate (70% cacao solids), chopped
• 1/4 cup (60 ml) hot espresso (or water)
• 7 ounces (200 g) butter, at room temperature, cubed
• 1 cup (135 g) flour
• 3 tablespoons unsweetened cocoa powder (the darkest you can find, natural or dutch-process)
• 1 1/4 teaspoon baking powder
• 5 large eggs, separated, at room temperature
• pinch of salt
• 1 cup (200 g) superfine sugar