In a small saucepan, heat the cream over medium heat until very hot, just below a simmer. Put the dark and milk chocolates into a medium bowl. Pour the hot cream over the chocolate and shake the bowl to settle it; let stand 2 minutes. With a small whisk, starting in the center of the bowl, whisk in small circles, working your way outward, until the chocolate is completely melted and the sauce is smooth.
Use warm or at room temperature. Store the leftover sauce in an airtight container for up to 1 week. The sauce can be warmed over low heat in a saucepan before using.