For the jicama coleslaw: In a medium bowl, combine the jicama, red onion, bell pepper, jalapeno, mayonnaise, buttermilk, lime juice, salt, sugar and cayenne. Mix well, then cover and refrigerate for at least 1 hour and up to overnight. Taste and adjust the seasoning if necessary just before serving.
For the chili shrimp tacos: Mix the oil, chili powder, salt, coriander and cumin in a large bowl. Add the shrimp and toss to coat. In a nonstick pan over medium-high heat, saute the shrimp just until opaque, 1 1/2 to 2 minutes per side.
Place the tortillas over an open flame until slightly charred and pliable, about 20 seconds per side. (Alternatively, wrap them in a damp paper towel and heat in a microwave.)
In a small bowl combine the avocado, lime juice and cayenne.
Divide 1 cup of the jicama coleslaw among the tortillas and then top each tortilla with 2 or 3 shrimp, a dollop of the avocado mixture and a sprig of cilantro.
Serve 2 tacos per person, with a lime wedge on the side.
If you don't have buttermilk on hand, you can make your own by adding 1 tablespoon distilled white vinegar to 1 cup whole milk. Stir to blend, then set aside until thickened and creamy, usually about 5 minutes. Use as needed. (Makes 2 cups.)