Black and White Cookies

• ¼ organic lemon, thinly peeled (peel only, no pith)
• 3 ½ oz sugar
• 3 ½ oz unsalted butter, soft, cut into pieces
• 2 oz homemade vanilla sugar (see Tip)
• 2 large eggs
• 2 oz whole milk
• 7 oz pastry flour
• 2 oz corn starch
• 2 tsp baking powder

Icing

• 14 oz confectioners sugar
• 2 oz water, plus extra to thin, if necessary
• 1 oz freshly squeezed lemon juice
• ½ tsp natural vanilla extract
• 1 oz Dutch-processed cocoa powder