Make the dulce de leche: Remove and discard the labels from the can of sweetened condensed milk. Place the can into a large pot, laying the can on its side. Fill the pot with enough water to completely cover the can and bring the water to a boil over high heat. Once the water is boiling, lower the heat to medium-low and cook the can of sweetened condensed milk on a simmer for  3 ½ hours. Add water if necessary, so that the can is completely submerged in water during cooking. Use tongs to carefully remove the can from the hot water and set it aside to cool down. Use a can opener to open the can and the condensed milk should have turned into caramelized dulce de leche. Scoop out the dulce de leche into a mixing bowl. Add the salt and vanilla extract, and whisk to combine thoroughly. Set aside.

Make the dough: Whisk the flour, cornstarch, baking powder and salt in a mixing bowl. Set aside. Add the butter and sugar into the bowl of a stand mixer fitted with a paddle attachment. Mix the butter and sugar at medium speed until it is light in color and fluffy, about 2 minutes. Add the eggs, vanilla extract and lemon zest into the bowl. Mix on low to medium low speed until combined. Add the flour mixture into the bowl with wet ingredients and beat until the dough comes together into a ball, about 1 minute. Place the dough onto a piece of plastic wrap, make it into a ball, then press it with your hands into a disk shape. Wrap the dough tightly and refrigerate for at least 45 minutes.

Preheat the oven to 350 F and line two baking sheets with parchment paper.

Shape and fill the hamantaschen: Transfer the dough from the refrigerator and discard the plastic wrap. Roll out the dough gently and on a lightly floured surface into a large circle that is  ¼ inch thick. Use a 3-inch round cookie cutter to cut out as many circles as possible. Remove the scraps of dough and set aside. Place a teaspoon of dulce de leche filling in the center of each circle of dough. Dip a finger into water and rub it around the border edge of the circles of dough. Pinch 3 edges of the circle of dough together creating three distinct points. Pinch the corners tightly, and up the sides very tightly, enclosing the filling. Repeat filling and shaping the remaining cookies. Re-roll the dough scraps and repeat the cookie shaping and filling process.

Bake the hamantaschen: Place the cookies onto the baking sheets about 1-2 inches apart. If you can, place the baking sheets with the cookies in the freezer for 15 minutes. This will help the cookies hold their shape. Transfer the cookies into the oven and bake for 12-15 minutes or until the edges are light golden brown. Remove the cookies from the oven and immediately sprinkle the cookies with the shredded coconut. 

Serve the hamantaschen at room temperature alongside a cocktail or an espresso cortado.