Line a large baking sheet with waxed paper. Place chocolate and butter in a bowl set over a pan of gently simmering water and allow to melt, stirring occasionally, about 2 minutes. Stir until smooth, then remove from heat.
Dip rounded ends of cookies in chocolate mixture, allowing excess to drip off. Arrange dipped cookies on baking sheet and refrigerate until chocolate has hardened, at least 10 minutes.