To make enough for 12 cocktails, start by pouring 4 cups boiling water over 2 cups fresh mint leaves. Let steep 10 minutes. Strain and cool. Meanwhile, in a 1-2 quart saucepan, bring to a boil 2 cups cranberries, 1 cup sugar, and 1 cup water over medium-high heat. Remove from heat and let cool, then strain.
To make one cocktail, combine 1/3 cup fresh mint tea, 2 tablespoons cranberry syrup, 1 tablespoon limoncello, 2 tablespoons vodka, and ½ teaspoon lemon juice in a cocktail shaker full of ice. Shake vigorously and strain into a martini glass. Garnish with a sprig of mint.