Place the cocoa powder on a small shallow plate. Moisten the rim of a martini glass with vodka or water. Dip the rim of the glass in the cocoa powder until coated all the way around.
In a cocktail shaker over ice combine the vodka and creme de cocoa and shake several times until well chilled. Pour through a strainer into the martini glass, and garnish with chocolate swizzle sticks if desired.