Preparation Steps

• Line two sheet pans with parchment paper and heat the oven to 360°F (182°C).
• Whisk together the flour, baking powder, and salt in a medium bowl; set aside.
• In a large mixing bowl, beat the softened butter, sugar, and lemon zest on medium-high until very light and creamy, 2 to 3 minutes.
• Beat in the egg, then the yolk, followed by the vanilla, scraping the bowl as needed.
• On low speed, add the dry ingredients in two additions, alternating with the milk (begin and end with dry). Mix just until no dry streaks remain; do not overmix.
• Let the dough rest for 10 minutes to hydrate. Meanwhile, prepare a large 2–3 tablespoon scoop and a small bowl of water for smoothing.
• Scoop 12–14 mounds onto the prepared pans, spacing about 3 inches apart. Lightly dampen your fingertips and smooth each mound into a gentle dome about 2 1/2 to 3 inches wide.
• Bake 12 to 14 minutes, rotating pans halfway, until the cookies are set and just barely golden at the edges while still pale on top.
• Cool on the pans for 5 minutes, then transfer to a wire rack. For a classic flat surface, flip each cookie upside down so the flat side faces up and let cool completely.
• Make the icings: In a bowl, whisk the powdered sugar, 1/4 cup milk, and vanilla until smooth and thick but spreadable. Scoop half into a second bowl and whisk in the cocoa, adding 1–3 teaspoons more milk until the chocolate matches the white icing in consistency.
• Using an offset spatula, spread white icing over one half of the flat side of each cookie. Let stand 10 minutes (or chill briefly) to set the first half.
• Finish with the chocolate icing on the other half, slightly overlapping the white edge for a crisp line. Allow cookies to set at room temperature for 30 minutes before serving.