Let the butter sit at room temperature until it’s soft enough to press into easily.
Add the butter to a stand mixer or large bowl with a hand mixer. Beat on medium-high speed until the butter turns pale and fluffy, about 4 to 5 minutes.
Drop the mixer speed to low and slowly pour in the milk (or buttermilk). Once it’s fully incorporated, increase the speed to high and whip for another 6 to 7 minutes until the butter is smooth and creamy.
Scrape down the sides of the bowl and whip one last time to make sure everything is well incorporated. The butter should look fluffy and have a glossy, shiny finish.
Transfer the whipped butter to a serving dish and sprinkle with flaky sea salt on top before serving.