Pure butter buttercream is delicious but unstable above 30°C. In a Chennai summer that means most of the year. Here is what we do, and what you can do if you are baking at home.
Recipe adjustment: replace 30% of the butter with a high-quality vegetable shortening (we use a specific brand we will share at our classes). The texture is nearly identical, the taste is 95% there, but the meltpoint is significantly higher. Pure butter cracks at 28°C; the blend holds to 34°C.
For a finished cake at home: refrigerate for 30 minutes before serving. The buttercream sets firm. Pull out, decorate the final touches (fresh flowers, gold leaf), serve within 90 minutes.
Transport: insulated cool box with one frozen gel pack placed at the bottom of the box, with a layer of cardboard between the pack and the cake. Direct contact with frozen packs will crack the buttercream. Set the box on the back seat of the car with AC on full.
Venue display: avoid sun-facing display tables. Even an indoor display by a window in May Chennai sun will start to soften the buttercream. Move the cake to a north-facing or shadowed corner of the venue.
Serving: cut and serve in batches of 20-30 slices. Keep the rest of the cake refrigerated, bring out as needed.
What actually fails: detailed piping (rosettes, leaves) starts to slump first. The flat areas of the cake hold up longer.