Prep your glass: Pour a thin ring of chocolate syrup around the rim of a martini glass. Dip the rim into a plate of cocoa powder or finely crushed dark chocolate, turning to coat evenly. Set aside.
Fill a cocktail shaker with plenty of ice.
Add the vodka, chocolate liqueur, crème de cacao, and heavy cream to the shaker.
Secure the lid and shake vigorously for 15–20 seconds until the outside of the shaker is frosty cold.
Strain the cocktail into the prepared martini glass. The drink should be smooth and silky with a slight frothy top.
Garnish with chocolate curls or shavings on the rim. If using, twist an orange peel over the glass to release the oils. Serve immediately.