Line a tray with waxed paper and set 24 mini silicone baking cups on it.

Melt the white chocolate chips in a heatproof bowl over simmering water, stirring until smooth. Remove from heat but keep over the pan.

Transfer melted white chocolate to a snack-size ziplock bag with a snipped corner and drizzle over the baking cups. Freeze until firm.

Combine dark chocolate and cocoa butter in another heatproof bowl over simmering water. Stir until melted and smooth. Remove from heat.

Working with one cup at a time, paint the dark chocolate on the bottom and sides of each frozen white chocolate cup, using about 1/2 teaspoon per cup. Freeze until firm.

Cover any thin spots or holes with more dark chocolate and freeze again.

Gently peel off the silicone cups and add your choice of fillings.