Add half of the flour mixture and beat on the lowest speed until just combined, about 20 seconds. Add the milk, 4 tablespoons lemon juice and 2 teaspoons vanilla extract, and beat on the lowest speed until just combined, about 10 seconds. The batter will appear curdled at this point. Add the remaining flour mixture and beat on the lowest speed until just combined, about 20 seconds. Divide the batter evenly between the wells of the muffin pan (a heaping 1/4 cup each), filling almost to the top.
