Heat the oven to 375°F and place a rack in the middle of the oven. Place the butter in a 9x5-inch loaf pan and put it in the hot oven. When the butter has melted, remove the pan from the oven.

Meanwhile, whisk together the flour, sugar, milk, and salt in a large bowl (it’s okay if the batter is a little lumpy). Pour the batter into the pan with the melted butter and use a spoon to lightly mix it. There should be visible streaks of melted butter that don’t get completely mixed; this yields the buttery, caramelized edges.

Place the blueberries on top of the batter in an even layer. Bake until the top is golden brown all over, 50 to 60 minutes. (If you remove it too early, the buckle will indeed “buckle” in the middle and fall—which would still be tasty, though sunken!) Wait until it’s golden all over the top and set in the center. Place the buckle on a cooling rack for 10 minutes before serving warm with a scoop of ice cream.