Step 1Combine the sugar with the water and a squeeze of lemon juice in a heavy saucepan. Stir to combine and cook over low heat until the sugar dissolves, 3 to 4 minutes. (The lemon juice helps stop the sugar from crystallizing.) Increase the heat to medium and boil the syrup rapidly without stirring until it starts to turn golden around the edges, about 10 minutes. Do not stir -- if you do, the syrup may crystallize.
Step 2Meanwhile, melt the butter with the cream in a separate saucepan over medium heat.
Step 3When the syrup begins to color, lower the heat a bit and continue boiling to a deep golden color, about 3 to 4 minutes; it will darken rapidly. Remove the pan from the heat and let it cool for 30 seconds. Add the butter mixture, standing back as the sauce will sputter and bubble up in the pan. Put it back over the heat, stirring until the caramel is completely dissolved. Let the sauce cool, then taste it and add a dash of salt to sharpen the flavor. Serve hot or chilled.