Those treats are very traditional in Argentina. Oh my goodness, they are delicious! Basically, it is almost like a biscuit sandwich filled with dulce de leche. Some versions of alfajores are coated with a chocolate sauce.

... For the Filling

... For the Dough

... For the Chocolate Coat

The Filling

Over a medium heat, in a medium saucepan, pour sweetened condensed milk and butter. Stir this mixture for 10-15 minutes until thicken. Place the saucepan on a cooling rack. Set aside.

The Dough

Pre-heat oven to 390 F. Line 4 baking sheets {13" x 18 "} with parchment paper. Grease with butter and sprinkle with all-purpose flour. Set aside.
Sift together all-purpose flour, baking soda and cornstarch. Set aside.
In a large bowl, put sugar, butter and beaten eggs. Using a wooden spoon, stir for 1 minute.
Add all-purpose flour, cornstarch and baking soda to the batter.
Using your hands, knead the dough until all is combined.
Sprinkle an even surface with flour. Divide the dough in four parts {it makes the work easier}. Place the dough over the sprinkled surface.
Using a rolling pin, roll out the dough about 1/2 inch thick.
Using a rounded cutter, cut the dough and arrange the biscuits on the baking sheets.
Bake for 20 minutes. Let them cool.
Spread the filling over on one side of the biscuit. Top with another. Repeat this procedure with remaining cookies.

The chocolate Coat

Line a baking sheet with parchment paper. Set aside.
Using a double boiler, melt the chocolate.
Dip the alfajores in melted chocolate and gently tap off the excess of chocolate. Place them over the lined baking sheet. Refrigerate for 1 hour. Make sure the chocolate coat is dry before you remove from fridge.