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1
Warm the limes in the microwave for 30 seconds to slightly soften them (this will make them easier to juice).
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2
Cut a thin slice off each end of the limes to make a flat base for each half to rest on.
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3
Cut the limes in half and squeeze out the juice into a medium bowl.
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4
Using a teaspoon or grapefruit spoon, scrape the remaining pulp and membranes out of the lime halves to make cups (discard the pulp).
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5
Transfer the cups to a plastic bag and refrigerate until ready to fill.
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6
Combine the water and sugar in a saucepan and bring to a boil.
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7
Set aside to cool.
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8
Puree the watermelon cubes in a food processor or blender, then strain through a coarse strainer into the bowl containing the lime juice.
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9
Stir in the vanilla extract and cooled sugar syrup.
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10
Pour into ice cube trays and freeze overnight.
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11
When ready to serve, turn the watermelon ice cubes into a food processor fitted with a metal blade or a powerful blender.
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12
Pulse the machine until the granita has the texture of crushed ice.
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13
Fill the lime halves very full with the granita, rounding the tops.
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14
Dot with the currants, making them look like watermelon seeds.
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15
Serve immediately.
