Preparation

• Position a rack in the center of the oven, and heat the oven to 375°F. Grease an 8-inch round cake pan with softened butter. Line the pan with parchment, and set aside.
• Put a large bowl over a pot of barely simmering water, making sure the bowl doesn’t touch the water. Add the butter, chocolate, and espresso powder, and heat, stirring occasionally with a silicone spatula, until the butter and chocolate are melted.
• Off the heat, add the sugars, whisking until incorporated. Whisk in the vanilla and then the eggs one at a time, breaking up each egg with a whisk before gently mixing it into the chocolate mixture. Pour the batter into the prepared pan, and bake for 30 to 35 minutes, rotating the cake after 15 minutes. The cake is done when the top is crackly and the center wobbles just the tiniest bit.
• Let the cake cool on a rack for about 10 minutes. Carefully invert the cake over the rack, then flip right side up to cool completely. The top of the cake will collapse slightly as it cools.
• Wrap the cake in plastic wrap, and refrigerate overnight. Serve slices at room temperature sprinkled with sea salt, a dollop of crème fraîche, and fresh berries, if you like.
• Alternatively, and if you are stretched for time, after the cake rests on the counter for 10 minutes, transfer it to a wire rack set inside a cookie sheet, and place in the freezer for 20 minutes. Then, invert the cake out of the pan and right side up onto a serving plate and serve warm, with the crème fraîche and berries, if you like.
• Store wrapped in plastic wrap at room temperature for up to 3 days.