Step 1

Preheat oven to 350 degrees F. Into large bowl, sift flour, cornstarch, baking powder, cinnamon and salt. With mixer on medium-high speed, beat butter and sugar until creamy. Beat in vanilla, then egg yolks, 1 at a time. With mixer on low, beat in flour mixture until just combined, stopping and scraping down side of bowl occasionally.

Step 2

On lightly floured surface, with lightly floured rolling pin, roll half of dough to 1/4-inch thickness. With 1 1/2-inch round cutter, cut out rounds. With small knife or mini offset spatula, transfer rounds to large parchment-lined cookie sheet, spacing 1 inch apart. Reroll scraps once. Bake 12 to 15 minutes or until golden brown on bottoms.

Step 3

Let cookies cool on cookie sheet 5 minutes. Transfer to wire rack to cool completely. Repeat rolling, cutting and baking with remaining dough.

Step 4

Make the Slow-Cooker Dulce de Leche: Transfer sweetened condensed milk to one 1-cup canning jar or three 13-cup jars. Stir in salt and seal tightly with lid; place in slow-cooker bowl (if using large jar, lay down sideways). cover with water. Replace lid on slow cooker and cook on low 8 hours. Carefully remove jar from water and wipe dry. Let cool 1 hour in refrigerator before opening. Makes about 1 cup.

Step 5

To assemble, place dulce de leche in piping bag fitted with star tip. Pipe onto half of cookies. Top with remaining cookies.

Step 6

Cookie sandwiches can be stored in airtight containers in freezer for up to 1 month.

Step 7

For the Hazelnut-Cocoa variation: Spread half of cookies with 1 teaspoon each hazelnut-cocoa spread instead of dulce de leche.

Step 8

For Fiesta Alfajores variation: After sandwiching, with small knife, spread dulce de leche all over sides of sandwich cookies. Roll in choice of sprinkles, flaked coconut, mini chips, or chopped nuts.