Soak the gelatin in a medium bowl of ice water until completely softened.
In a medium pot, bring the milk and cream to a simmer.
Squeeze the water from the gelatin, add to the pot, and stir to dissolve.
Place chocolate in a heatproof bowl and pour the hot milk/cream mixture over the top. Use a hand mixer to blend until completely smooth. Add the butter and blend until smooth. Season with flaky sea salt and transfer to a large bowl (or pastry bag). Place in the refrigerator to chill completely, at least 3 hours.