Preheat the oven to 350 degrees farenheit.
In a stand mixer bowl (or other large bowl), cream together your butter, shortening, granulated sugar and powdered sugar for about 2 minutes till light and fluffy.
Add in the milk and eggs and stir till combined.
Add in the flour, baking soda, cream of tartar and salt and stir till it all comes together. The dough should come together, but be a little crumbly.
Scoop the dough into 3-4 TBS sized scoops and roll into balls.
Place 8 at a time on a baking sheet lined with a silicone liner or parchment paper.
Add remaining 1/4 cup granulated sugar to a small flat plate.
Spray the bottom of a flat circular glass cup with nonstick spray. Press it into the sugar.
Press each cookie down using the glass cup until they're about 1/2 inch thick.
Bake in the preheated oven for 8-10 minutes until the cookies look barely done on top. They may not look cooked through the middle, but they will keep cooking on the tray.
Allow cookies to cool for several minutes on the baking pan. Then remove them to a cooling rack to cool completely.