Add the grapefruit juice, sugar, water and sliced chilli to a small saucepan. Place on a high heat, bring to the boil, stir and simmer for 10 minutes. Turn off the heat and leave to cool before removing and discarding the chilli.
Add the lime juice to the chilli-grapefruit mixture.
Take the grapefruit slices and dust with the brown sugar. Use a cooks blowtorch to carefully caramelize and slightly char the sugar and grapefruit on both sides (or you could place under a very hot grill or place on a griddle to caramelize).
Fill three glasses with the crushed ice. Add 3 slices of lime to each glass. Divide the chilli-grapefruit liquid between the three glasses and top with ginger beer.
Make a small slit in the grapefruit wedges and place on the side of the glasses for decoration. Top each glass with a Thai chilli and serve.