In a large saucepan, heat the cream and instant espresso powder over medium heat, stirring occasionally, until the mixture bubbles around the edges, 2 to 3 minutes. Remove from heat, add the semisweet and bittersweet chocolate, and stir until melted. Add the liqueur (if using) and vanilla and stir to combine. Transfer to a shallow bowl, cover loosely, and refrigerate until firm, at least 4 hours and up to 1 week.
Using a small spring-loaded scoop, a melon baller, or a teaspoon, drop 1-inch balls of the chocolate mixture onto a wax-paper-lined baking sheet. Refrigerate until firm, about 15 minutes.
Place the cocoa powder in a shallow bowl. Roll the chocolate balls in the cocoa, then transfer to a wax paper-lined baking sheet and refrigerate. Bring to room temperature before serving.
Makes 48 truffles