Whisk the egg, vanilla and lemon juice in a small bowl and set aside.
Using an electric stand mixer, or hand mixer, cream butter until fluffy and scraping down the sides of the bowl with a rubber spatula. Add sugar and continue beating until light, about 2-3 minutes. Then add egg mixture, beat for about 1 minute.
Combine flour, baking powder and salt in a medium bowl. Then add to the butter mixture, and mix until well blended.
Divide the dough in half and shape each into a large disc. Wrap each disc tightly in saran wrap and refrigerator until firm, about 2 hours or overnight.
Preheat oven to 350°F and line baking sheets with parchment paper.
Take one disc out of the refrigerator at a time and on lightly flour surface, roll out to ¼ inch thickness. Place 1 inch apart and repeat. Bake for about 12-15 minutes until light golden on bottom and edges. Let cool on baking sheets for about one minutes, then remove with metal spatula and place on cooling rack.
Melt chocolate in the microwave, and check every 20seconds, stirring each time. When most of the chunks have melted, remove and continue to stir until all the chunks are gone.
Once the cookies are no longer warm, dip half of the cookie into the chocolate and place on parchment paper and sprinkle with lemon zest over the melted chocolate. Place on parchment or wax paper to dry. Once all cookies are finished, leave at room temperature to harden.
Store in the refrigerator in an airtight container for about 1-2 weeks. These cookies can be stored in the freezer for about 2 months, in a sealed container.