DO-AHEAD TIP: Prepare cauliflower.
Melt half of the butter in a large skillet over medium-high heat; add garlic and onion and sauté for 2 minutes or until onion starts to “sweat” and becomes translucent.
Place cauliflower in a blender; add garlic/onion mixture and process until smooth, gradually adding the broth and milk until fully combined.
Transfer mixture to a large saucepan and add seasonings (salt through cayenne pepper); set aside on low heat to simmer.
In the same skillet, melt remaining butter over medium heat; add shrimp and lightly season with salt and pepper; cook for 2 to 4 minutes per side or until pink and opaque. Add to soup and serve.