Cut the habanero pepper in half. Remove seeds if you don't want full spice (I kept them in).
In a mason jar or other airtight container, combine the habanero slices and tequila and let steep for at least three hours. The longer you steep, the spicier it will get.
Slice all three grapefruits in half and place five of the halves on a baking sheet. Put them under the broiler for about 10 minutes, until their skins are roasted and shriveling and flesh is charred.
While grapefruit is roasting, cut remaining grapefruit half into slices. Slice lime.
Remove the grapefruit and let them cool. When you are able to touch them, juice all five slices and place juice in the fridge to cool completely.
When tequila has steeped and grapefruit juice has cooled, combine 4 oz. grapefruit juice, 1 ½ oz. habanero tequila, ½ oz. grand marnier, and 1 oz. lime juice in a shaker.
Pour into ice-filled glass rimmed with mixture of sea salt and cayenne. Add sliced grapefruit and lime.
Repeat with remaining tequila and grapefruit juice.