Sift all the flour. Measure out the first 4 cups and mix it with the baking powder and salt.
Cream butter and sugar. Add eggs, one at a time, then add milk, zest, vanilla to combine. Change the whisk attachment to a paddle if you're using a stand mixer. Add dry ingredients to liquids incrementally. Work the paddle attachment on medium speed, and add enough of the additional 2 cups of flour to form a pliant, but firm, dough that doesn't stick. Knead with the paddle attachment, adding more flour as needed, until smooth and soft.