Put the sugar in a heavy bottomed pan. Mix in a couple of tbsp. of water, just enough to wet the sugar. Set on medium heat. Let the sugar dissolve/ melt. As it does, it will gradually deepen in colour.
Stir occasionally until the sugar is fully dissolved, and is an amber colour. Take the pan off the heat. Whisk in butter carefully as the mixture will spit. Then immediately stir in cream and salt, if using more. The mixture will be a bit thin in consistency. It will thicken as it cools.
Let it cool completely before using for the buttercream. If not using immediately, transfer to a bottle.
For the Devil’s Food Cupcakes
Lightly whisk together the cocoa powder, flour, corn-starch, salt, baking soda, and baking powder together in a bowl.
In another bowl, with a hand held mixer, beat the butter and sugar until creamy. Then beat in the eggs, one at a time, till smooth. In a small bowl, dissolve the instant coffee powder in the hot water. Then mix in the milk.
Add half of the flour mixture and the milk to the butter-suagr mixture and beat on slow to medium speed till just blended. Mix in the remaining flour mixture. Do not over beat the cake batter or you will end up with a dense tough cake rather than an airier one.
Divide the batter equally between 18 cupcake paper cups or tins. Bake them at 180C (350F) for about 15 to 20 minutes. It is difficult to judge if these are cooked from the colour. Over baking will affect the texture. Once the top of the cupcakes look set check for doneness with a skewer through the middle after about 12 to 15 minutes in the oven.
Cool the cupcakes on a rack. Let them cool completely before frosting.
For the Salted Caramel Frosting :
Beat the butter, using a hand held mixer, till light and fluffy. Beat in the cream and all the Salted Caramel. Beat in the icing sugar, a little at a time, until the frosting is of reasonably stiff consistency. Check for salt and beat in salt to taste. Frost the cupcakes as desired.