Combine instant coffee, sugar, and hot water (plus salt and vanilla if using) together with a rubber spatula.
Whip with a hand mixer until fluffy. Start low, then turn to medium speed. Whisk for about 3 minutes, it will look lighter in color. Make sure everything is incoprorated with the rubber sptaula.
To make the cocktail:
Place one large cube of ice in a whiskey glass.
Pour all liquid ingredients together over ice and stir.
Place Dalgona Coffee Whip on top with a spoon. Sprinkle with chocolate shavings.