Combine the butter, onion, garlic, and red pepper flakes in a large soup pot over medium heat, stirring regularly for a couple of minutes until the onions soften.
Pour in the vegetable broth and the lentils and simmer, covered, until the lentils are tender; start checking after 20 minutes, but keep it mind that it may take as long as 50 minutes.
While the lentils cook, heat the 3 tablespoons of butter in a small saucepan over low heat. Stir it constantly with a rubber spatula until it starts to foam and brown, about 5 minutes. Stir in the curry powder and sauté until the mixture is very fragrant, just 30 seconds to 1 minute. Set aside.
When the lentils have finished cooking, take the pot off the heat and stir in the coconut milk and 1/4 teaspoon of sea salt.
Purée the soup using an immersion blender, or transfer it in batches to a blender or food processor. I left the soup slightly chunky, but you can also purée it until it's very smooth (and strain it if you're looking for an even silkier texture).
Stir in half of the curried butter and taste the soup for salt, adding more as needed. Serve the soup drizzled with the remaining curried butter and with cubes of pan-fried paneer, if you like.