2) Remove from the heat and leave to steep until cool. Strain and reserve the simple syrup.
For the cocktail:
1) Meanwhile, heat the griddle to high. Lightly brush the watermelon slices with vegetable oil and season with salt. Arrange on the griddle and cook until you have visible marks on both sides.
2) Remove the rind and chop the flesh into large chunks. Add the watermelon flesh to a blender, along with the water, and blend until liquefied.
3) Strain the juice, through a fine strainer, into a bowl. Discard the pulp and reserve the watermelon juice.
4) In a large bowl or glass jug, combine the tequila, simple syrup, lime juice and watermelon schnapps. Add the grilled watermelon juice, and stir.
5) Pour into serving glasses and garnish with the reserved slices of grilled watermelon and a jalapeno chilli pepper.