My oven takes a while to get to the correct temperature so I turn my oven on as soon as I start putting ingredients together. In a bowl either measure or weigh the flour, baking powder, and salt together, (I run a whisk through them to blend) then set the bowl aside.
Cut the ends off of the lemon or lemons if using small and zest (I use a Microplane Zester) into a small bowl. Don't go very deep, you just want the yellow, the underlying white starts to get bitter. Then cut the lemon in half and juice your lemon into another small bowl. I use a Chefn Juicer. Set both small bowls aside.
In a stand (I use a KitchenAid) or hand-held mixer bowl blend the sugar and butter together. I use medium speed for about 2 minutes. Add the egg and vanilla extract and mix for another 2 minutes. You want to blend these ingredients. I scrape down the bowl and finish blending.
Add half of the flour, baking powder, and salt mixture and blend till incorporated. I stop the mixer and scrape down the sides. Add the milk and blend for about 30 seconds before adding the rest of the flour mixture. Make sure all the flour is mixed into the rest of the batter. The cookie dough is very soft so I stick the bowl into the refrigerator for about half an hour. The dough will still be very soft but the butter will have to stiffen up a bit.
Line 2 cookie pans with parchment paper and using a small cookie scoop (I use a OXO Good Grips Cookie Scoop, Small) put 12 small mounds onto the cookie sheet. Bake at 375° for 11-13 minutes. My cookies were done at about 12 mnutes. Leave them on the pan for about 1 minute before moving to a cooling rack.