In the mixer bowl, whisk the warm water, yeast, and 1 teaspoon of sugar. Let stand until foamy, about 5 minutes.
Add ¾ cup of cold milk, the rest of the sugar, and egg. Whisk to combine.
Attach the bowl of the mixer and fit with the paddle attachment. Mix on low speed, adding 1 cup of flour and the salt. Beat in butter 1 tablespoon at a time. Add in the rest of the flour or until the dough cleans the side of the bowl.
Remove the paddle attachment and attach the dough hook. Knead on medium-low speed for 5 minutes.
Transfer the dough to a medium bowl lightly sprayed with cooking spray and cover with plastic wrap. Set in a warm spot until doubled in size, about 40 minutes.
Using a kitchen scale, divide and weigh the dough into 12 (1-ounce portions). Gently roll the dough pieces into balls and place them into a parchment-lined air fryer basket.
Loosely cover with plastic wrap and let rise at room temperature for 40 minutes.
Bake at 300 degrees for 10 minutes or until golden brown.
Brush them with the melted butter and sprinkle with the Maldon salt. Serve warm.