1. Toss shrimp with kosher salt and 3/4 teaspoon coarsely ground pepper.
2. Heat butter in a 12-inch heavy skillet over medium-high heat until foam subsides, then sauté shrimp, turning, until pink and just cooked through, 3 to 5 minutes.
3. Reduce heat and add tequila, then increase heat to medium-high.
4. **BE CAREFUL WITH THIS**
5. Tilt skillet over gas burner to ignite tequila (or ignite with a long match; use caution, as flames may shoot up high).
6. Cook, shaking skillet gently once or twice, until flames subside.
7. Remove from heat and stir in crema.
8. Serve sprinkled with scallion.