1. Preheat the oven to 325°F (163°C).
2. Combine the coconut, sweetened condensed milk, lemon zest, lemon juice and vanilla extract in a large bowl. Set aside.
3. Add the egg whites and salt to a large mixer bowl and whip on high speed with the whisk attachment until they reach stiff peaks.
4. Carefully fold the egg whites into the coconut mixture.
5. Drop rounded tablespoons of the coconut mixture onto a cookie sheet lined with parchment paper or a silicone baking mat.
6. Bake for 25 to 30 minutes, until lightly brown. Allow to cool.
7. Melt the white chocolate according to the instructions on the package.
8. Dip of the bottom of the macaroons into the chocolate and place on parchment paper to dry.
9. Drizzle the remaining chocolate over the tops of the cookies, then sprinkle with additional lemon zest, if desired.
10. Store in an airtight container at room temperature.