Prepare the passion fruit juice by scooping out the flesh from 2-3 large ripe passion fruit. Place the pulp into a strainer and press out all the juice using the back of a spoon. You should end up with 25ml.
Add this to a high-speed blender along with the tequila, Cointreau, lime juice, frozen mango, agave nectar and a small handful of crushed ice. Blend for about 1 minute, or until smooth.
Mix together the sea salt and lime zest and tip onto a small side plate. Wet the rims of the serving glasses with a little lime juice and tip the glasses upside-down onto the salt. Spin the glasses until you have an even rim of lime salt.
Fill the glasses three-quarters full with crushed ice. Pour over the margarita and garnish with half a passion fruit.