Whisk together the AP flour, white sugar, baking powder, and kosher salt in a large bowl.
In a smaller bowl, whisk together the egg and milk until frothy and pale yellow.
Cut the softened butter up into small pieces.
(If using a stand mixer, attach the dough hook and combine the dry and wet ingredients together. Once combined, incorporate the softened butter.)
If mixing dough by hand, using a whisk, make a well in the dry ingredients.
Pour the milk and egg mixture into the well.
Incorporate the dry into the wet ingredients by pulling the dry ingredients slowly into the center. Be patient with this step!
Once incorporated, roll out dough onto a floured work surface and knead with floured hands.
While kneading, feel free to incorporate more flour. Up to ½ cup, but no more.
Knead in the softened butter. At this stage, the dough will feel too wet and greasy, but just continue to work with it!
Roll the dough into approximately an 8 x 10 inch rectangle.