4-5ouncesheavy cream, lightly whipped (optional, see note)
Fill two lowball glasses with ice cubes.
Add 2 ounces vodka and 2 ounces Kahlúa to each glass.
Top each drink evenly with lightly whipped heavy cream (see note). If you want to have visual separation between the layers, pour the whipped cream over the ice cubes or a spoon to avoid having it sinking down too quickly.
Whip the cream very lightly until it just starts to become a bit more dense. You can add it without whipping but it will mix with the other ingredients faster.
Substitute half and half or whole milk. Milk won't float as nicely but it will still taste nice.
Make a Mudslide by omitting the cream, and substituting one ounce of Irish cream liqueur in each drink.