For the spicy chipotle crema: In a blender, add the sour cream, milk, olive oil, garlic powder, salt, pepper and chiles and blend until combined. You can always add more peppers for a hotter sauce. Set aside.
For the tostadas: In a large skillet over medium-high heat, melt the butter. Add the garlic and onion and saute until translucent. Season the shrimp with salt and pepper and add to the skillet. Cook until the shrimp start to change to a pinkish tone, 3 to 5 minutes. Add the tequila to the skillet and tilt the skillet slightly over the burner to ignite it. Remove the skillet from the heat and let the flames subside, then return it back to medium-high heat. Add the chipotle crema, bring to a simmer and cook until fully combined and warm, about 2 minutes.
Divide the shrimp mixture among the tostadas and sprinkle with the cilantro.
Use extreme caution when igniting alcohol. Remove the pan from the heat source before adding the alcohol. Pour the alcohol into the pan and carefully ignite with a match or click lighter. Return the pan to the heat and gently swirl to reduce the flames.