All buttercream will melt eventually if subject to warm temperatures, however you can improve the resilience of your buttercream by substituting some of the butter for vegetable shortening. Vegetable shortening has a higher melting point than butter, so adding it to the buttercream will improve the frosting's overall heat tolerance. Although it has better heat performance. vegetable shortening doesn't taste nearly as nice as good quality butter, so at most I recommend substituting only half the butter with equal amount of shortening.