Chill your martini glasses in the freezer for 10-15 minutes. If short on time, fill each glass with ice water for 2-3 minutes, then discard before pouring.
Pour chocolate sauce onto a flat plate. Dip the rim of each chilled glass into the sauce, rotating to coat evenly. For extra elegance, dust the rim with unsweetened cocoa powder after coating.
Using a spoon or squeeze bottle, drizzle chocolate sauce in a spiral pattern inside each prepared glass, tilting to let it cascade down the sides. Set aside while you prepare the cocktail.
Fill your cocktail shaker halfway with ice cubes. Do not use crushed ice as it melts too quickly and can dilute the drink.
Measure and pour 3 oz vodka, 2 oz chocolate liqueur, 1 oz Irish cream, and 1 oz heavy cream into the shaker over the ice.
Secure the lid firmly and shake vigorously for 15-20 seconds until the shaker feels very cold in your hands, indicating it's properly chilled and aerated.
Open the strainer and pour the cocktail into prepared martini glasses in a slow, steady stream, filling each about two-thirds to three-quarters full.
Garnish with a maraschino cherry, a light dusting of cocoa powder, or chocolate shavings. Serve immediately while still cold and frothy.