Chop one of the chillies and, together with the tequila, place in a small bowl. Cover with plastic wrap and set aside to infuse overnight. Strain the tequila into a large bowl and add the lime juice, Grand Marnier and mango juice. Stir to combine and chill. Place the salt on a fl at plate or surface. Rub a lime wedge around the rim of each glass before turning it upside down onto the salt and then twisting the glass to coat the rim. Serve the margarita in a Martini or wine glass with crushed ice and garnish with the remaining chilli and lime wedges.
Cook’s note: To serve this as a frozen margarita, blend the mixture with some ice until it is slushy.