In a bowl, work the softened butter with the sugar until a smooth, even cream forms.

Add the egg yolk, then the whole egg, and whisk with electric beaters until the mixture is light and pale.

In another bowl, sift the flour and potato starch, add the lemon zest and mix.

Gradually incorporate the dry powders into the butter and egg mixture until a soft dough forms.

Transfer the dough to a lightly floured surface, work it by hand, form a flat disc and let it rest in the refrigerator for 2 hours.

For the dulce de leche, put the milk, vanilla extract, sugar and a pinch of baking soda in a saucepan.

Cook over low heat, stirring, until the cream becomes thick and amber-colored.

Pour into a bowl, cover with plastic film and let cool.

Roll out the dough to half a centimeter thick, cut out discs of about 4 cm and arrange them on a baking tray lined with parchment paper.

Bake in a fan oven at 180°C for 12 minutes, then let cool.

Fill one biscuit with half a teaspoon of dulce de leche, close with another biscuit, and roll the edges in grated coconut.