In a large bowl, combine the flour and salt. Dissolve the yeast in warm milk with the sugar, and let it foam for about 5 minutes. Then, mix everything together until a rough dough forms.

Roll the cold butter into a rectangle. Incorporate it into the dough by rolling and folding it several times to create those magical layers.

Wrap your dough in plastic wrap and refrigerate it for about an hour.

In a bowl, mix the lemon zest, juice, and sugar until fully combined.

Roll out the chilled dough and cut it into triangles. Place a spoonful of the lemon filling at the base of each triangle, then roll them up tightly.

Preheat your oven to 400°F (200°C). Arrange the croissants on parchment-lined baking sheets and bake for 15-20 minutes, or until they are beautifully golden brown.