Preheat oven to 400 degrees F. Grease and flour a large baking sheet and reserve.
Combine eggs and sugar in a large bowl and, using a hand mixer, mix until pale and fluffy, about 5 minutes. Add the baking powder and continue mixing until incorporated.
Using a wooden spoon or a hand whisk, carefully add the potato starch, a little at a time, being careful not to deflate the batter.
Transfer the batter to the prepared baking sheet and bake until puffed and golden brown, 8 to 10 minutes.
When done, remove from oven and let the cake cool slightly. Then, using a small round cookie cutter, cut about 20 discs (depending on the size of your cutter).
Spread each disc with dulce de leche (about 1 tablespoon). Top with another disc, forming a sandwich cookie. Repeat with the remaining discs.
In a small saucepan, bring 1/2 cup water to a boil. Once boiling, add the sugar and let it simmer until the sugar is dissolved. Remove from heat and, using a fork, dip each bem casado for several seconds per side in the sugar glaze, placing them immediately on a cooling rack to dry for at least 30 minutes.
Serve them right away or wrap them individually to give as gifts or party favors.