For the chocolates with brittle, line any ice cube molds (stars, hearts, shamrocks, etc.) with a little fat.
Melt the milk chocolate in a water bath and divide among the ice cube molds. Sprinkle hazelnut brittle on top of the melted chocolate. Then divide the melted white chocolate among the molds as well (to a smaller proportion).
Let the chocolates cool until the chocolate hardens again.
Carefully remove the pralines with brittle from the molds.