Make sure to keep the buttermilk very cold. Store it on ice until you're ready to serve.
Place potatoes in a steamer. Fill a large pot with 1 to 2 inches water; place steamer in pot. Bring water to a simmer, cover, and steam until potatoes are tender and easily pierced with a knife, 20 to 25 minutes.
Meanwhile, heat oil in a small skillet over medium. Add fennel; season with kosher salt. Cook,stirring occasionally, until fennel is starting to caramelize, 8 to 10 minutes. Add shallot; cookuntil golden brown and caramelized, about 5 minutes more.
Divide potatoes evenly among 4 bowls. Pour buttermilk over potatoes. Top each bowl with fennel mixture, kale, and dill. Season with flaky salt and pepper; serve.
For a special presentation, pour the ice-cold buttermilk over the warm potatoes at the table.