Spread the sugar evenly in the bottom of a small, heavy-duty saucepan. Pour the water evenly over the sugar. Do not stir. Let it sit, covered, for 30 minutes or so, until the sugar absorbs the water.
Swirl the pan gently over medium heat until the sugar is dissolved and clear, still not stirring. If sugar starts to crust around the sides of the pan, brush it down gently with a clean pastry brush dipped in cold water.
Cover the pan tightly and increase the heat to medium-high. Allow it to boil for 2 minutes.
Uncover and continue boiling until the syrup begins to darken around the edges, still not stirring. Swirl the pan gently and continue cooking until the syrup is uniformly amber in color. Don’t allow it to burn, or it will become bitter. Remove from the heat, add the butter and stir until smooth.
Add the cream and stir until completely smooth, then add chocolate and stir until smooth and shiny. Stir in salt and vanilla until smooth. Pour the mixture into a glass bowl or measuring cup and stir occasionally until it is the desired consistency. The ganache can be refrigerated in glass jars for at least a couple of weeks.
For icing: Either pour the still-warm ganache over a cake or brownies for a smooth glaze, or allow it to cool and thicken until it is a spreadable consistency.
For truffles: Refrigerate the ganache in a glass bowl for 2 hours. When it is firm enough to hold its shape but still malleable, scoop the ganache into half-inch balls and roll in cocoa powder.
For chocolate mousse: Chill a medium bowl and beater in the refrigerator. Allow the ganache to cool to just lukewarm. Pour the ganache into the chilled bowl and whip until it thickens. Spoon into stemmed glasses or serving bowls and chill until just before serving. Garnish with whipped cream.
For Spoon Fudge: Pour whatever ganache you’re not using for icing or mousse into a glass jar with a tight-fitting lid. Keep it in the refrigerator and have a spoonful as the mood strikes.