In a pot, whisk together the egg yolks. Pour in the cream and whisk until well combined. Add the sugar, a pinch of salt, and espresso, and whisk again. Heat over medium heat, stirring continuously until it thickens, about 5-8 minutes. Once it's thick enough to coat the back of a spoon and doesn't move, remove it from the heat. Make sure to do this over medium heat, not high, to avoid scrambling the eggs. Turn off the heat once it's thickened.