In a large mixing bowl, sift together the flour and cocoa powder. Add in your chopped butter and, with your fingertips, rub the butter into the flour/cocoa mixture until it resembles breadcrumbs. Stir in the sugar, cardamom, and lemon zest. Then, add your milk. I like to start with 2 Tbsp, stir everything with a butter knife, and add 1 Tbsp at a time until everything begins to come together. When you have a bunch of clusters as opposed to crumbs, use your hands to press the dough into one piece. Wrap in plastic wrap and let rest in the refrigerator for 30 minutes.