Prepare the dough: In a food processor, place the butter, flour, egg, and salt and pulse to create a sandy consistency. Press the sandy mixture into a dough without overworking it. Place the dough between two sheets of parchment paper and roll it into a circle about 14-inches across and ⅛-inch thick.

Transfer the dough to a 10 or 11-inch tart pan, press the dough to the sides of the pan and cut off the excess dough. Using a fork, prick all over the bottom of the tart. Place the tart pan in the freezer for 30-45 minutes or in the fridge for 1 hour. The dough should be very cold when baked. 

Preheat the oven to 350°F.

Place a piece of parchment paper over the pie dough and fill with pie weights, dried beans or rice. Bake for 15 minutes, until set. Remove the parchment and weights and bake for another 5 minutes, until golden.  

Meanwhile, prepare the coconut layer: In a large bowl, mix together coconut, egg, sugar, and milk and set aside. 

Remove the tart base from the oven and let it cool slightly, about 5 minutes. 

Spread the dulce de leche evenly over the base. Use a wet spoon to level it.

Gently distribute the coconut layer over the dulce de leche, using a wet spoon or your hands to level it without pressing too much.

Bake for 20-30 minutes more, until golden on top. Let cool completely before slicing.