How to Make It
Preheat oven to 350°F.
Sift together flour, baking powder, and salt in a medium bowl and set aside.
Combine milk and vanilla extract. Set aside.
Using a stand mixer or electric beater, cream butter and sugar for 2-3 minutes on medium speed until fluffy and creamy. Scrape down sides of mixing bowl with a spatula. Beat in egg and lemon zest for approximately 30 seconds. Reduce mixer to low speed. Add the flour mixture and milk mixture alternately to batter, beginning and ending with flour mixture.
Using a medium-sized ice cream scoop or ¼ cup measuring cup, scoop out approximately 2-ounce portions dough onto parchment-lined baking sheet, spacing the portions 2’’ apart. Bake on middle rack until cookies are set golden brown on top, about 15-17 minutes. Set on a cooling rack and allow to cool completely before icing.
While cookies cool, whisk together the confectioner’s sugar, corn syrup, and 1 tablespoon of hot water in bowl until smooth using an electric mixer or whisk. Add additional hot water, teaspoon by teaspoon, until desired consistency is reached. Stir in vanilla and lemon juice. Reserve half of icing and set aside. For the chocolate icing, melt the bittersweet chocolate in a saucepan set over low heat (or in the microwave on high for 1 minute). Stir until smooth. Add the melted chocolate mixture to 1/2 of the vanilla icing mixture and stir until smooth. If the mixture is too thick, reheat over low heat and add additional hot water, teaspoon by teaspoon, until desired consistency is reached.
Using a small offset spatula or knife, ice one half of each cookie with vanilla frosting. Run the spatula along the edge of the cookie to scrape off excess frosting. Place the cookies on a wire rack with parchment paper underneath, and allow excess frosting to drip off. Let the white halves of each cookie fully set. (Pro tip: To quicken the setting of the glazes, place the cookies in the refrigerator for about 15-20 minutes.) When vanilla icing has set, ice remaining half of each cookie with chocolate fondant icing. If the mixture is too thick, reheat over low heat. The mixture is easiest to work with, and pours smoothly, at about 100°F.
Allow frosted cookies to set an additional 20-30 minutes before serving or packaging.