This cocktail can be constructed right in the serving glass, so no need for a cocktail shaker or any extra supplies.
In an old fashioned glass filled with ice cubes, pour in equal parts coffee liqueur and then vodka.
Top with soda water.
Notes
If you want to maintain distinct layers in your drink, begin with Kahlua, then pour vodka slowly over the back of a spoon and then pour the sparkling water to top very slowly. Use the back of a spoon to minimize disturbance between each layer.
For a sweeter variation, you can swap the club soda for lemon-lime soda or ginger ale.
Mind Eraser Shot - You can enjoy this recipe in shot form. Make it by layering half an ounce of each ingredient in a shot glass.
Black Magic - Replace the fizzy water with lemon juice to create a Black Magic cocktail.