For the Simple Syrup:

In a small saucepan, combine equal parts sugar and water.
Heat over medium heat, stirring until the sugar dissolves. Let it cool before using.

For the Spicy Rim:

On a small plate, combine salt, chilli powder, and cayenne pepper to taste.
Rub a lime wedge around the rim of a martini glass, then dip the rim into the spicy mixture to coat.

For the Cocktail:

Fill a cocktail shaker with ice.
Add vodka, guava juice, freshly squeezed lime juice, simple syrup, and hot sauce.
Shake vigorously until well-chilled.
Strain the mixture into the prepared martini glass.
Garnish with a slice of guava.