Position the racks to divide the oven into thirds and preheat the oven to 350° .
Line two baking sheets with parchment paper.
Whisk together the flour, baking soda, baking powder and salt.
Beat the butter on medium speed for a minute or two, until smooth and creamy. Add the dulce de leche and beat for another minute.
Add the sugars and beat for 3 minutes more. Add the egg, beating for 1 minute more.
Scrape down the sides and the bottom of the bowl and, on low speed, add the dry ingredients, mixing for about 2 minutes. The dough will be very soft and mushy.
Pour the ½ cup of sugar onto a small plate. Roll the dough between your palms into balls and drop the balls, a couple at a time, into the sugar.
Roll the balls around in the sugar to coat them, then place on the baking sheets, leaving 2 inches between them.
Dip the fork in the sugar and press the tines against each ball until you have a flattened round.
Bake for about 12 minutes, rotating the sheets from top to bottom and front to back at the midway point. The cookies should be a little soft.
Let the cookies rest on the baking sheets for one minute before transferring them to the cooling racks. Cool to room temperature.