In a large mixing bowl combine the softened butter, sugar, egg and vanilla extract. Using a hand mixer, whisk the mixture until all the ingredients combined well.
Next, add the baking powder, salt and about half of the flour. Using a spatula start mixing the dough. Add the rest of the flour and knead the dough. The dough should turn out soft and not stick to your hands.
Next, slightly flour the work surface, place the dough on top and roll it into about a 1/4-inch-thick layer.
Using a cookie cutter, cut out the cookies. I used a circle cookie cutter 2-inch in diameter. The dough is very pleasant to work with, and the cookie circles hold the perfect shape. Transfer the cookies to a baking sheet covered with parchment paper or a Teflon sheet and bake at 350 degrees Fahrenheit for about 15 minutes or until they have a light golden-brown color. Let them cool completely before making cookie sandwiches.
To assemble the cookies, add about a teaspoon of dulce de leche filling over the inner surface of the cookie (the part that was stuck to the baking sheet). Cover it with the other half and slightly press them together. Using an extra filling, spread it well to the edges of the cookies and roll each stuffed cookie in the coconut mix or chopped nuts until it sticks to the dulce de leche filling. Repeat the process with all the cookies.