Place the sugar and water in a heavy saucepan over medium-low heat, whisking occasionally until the sugar is completely dissolved, about 3 minutes. After the sugar is dissolved, raise the heat to high and bring the mixture to a boil, undisturbed, until the syrup turns a deep golden brown, about 15 to 20 minutes. If you see the sugar caramelizing only in one corner you can gently tilt or rotate the pan to distribute the color evenly, but do not whisk. When the syrup has colored nicely, remove the pan from the heat and immediately add the cream. Be careful and stand back because the mixture will sputter. Do not stir until the mixture settles. Return the pan to low heat and stir with a wooden spoon until the strands of caramel melt. Remove from the heat and stir in the espresso and the butter. Serve warm.
Note: Tom likes to caramelize the sugar dry (without adding the 1/3 cup water to dissolve it) in a heavy pan over high heat, but many people find it easier to dissolve the sugar in a little water first. This caramel sauce is also delicious without the espresso.