Submerge a can of condensed milk in a pot of water and boil for 3 hours. Ensure the can remain fully submerged at all times. Cool completely before use.
Beat butter, sugar, lime zest, vanilla, and salt until creamy.
Mix in the egg yolks.
Add cocoa powder, flour, corn flour, and baking powder. Knead into a cohesive dough, wrap in plastic, and chill for at least 1 hour.
Preheat your oven to 175°C. Line baking trays with parchment paper.
Roll out the dough on a floured surface to 4–5 mm thickness. Work with small portions to avoid tearing.
Cut out rounds (4–5 cm in diameter) and place them on the prepared tray.
Bake for 12 minutes until the edges are light golden brown. Cool completely on a wire rack.
Pipe a dollop of dulce de leche onto half of the cookies. 
Sandwich them with the remaining cookies. Roll the sides in grated coconut for the perfect finish.