In the bowl of a mixer, beat the butter along with the granulated sugar at medium speed for a couple minutes until smooth, creamy, and puffed up. Add the vanilla and the eggs one at a time, beating until well incorporated.
In a bowl combine the flour, baking powder, and salt.
With the mixer running at low speed, add the flour mixture and milk in multiple additions, alternating between the two. Continue beating until well incorporated and you have a homogenous dough. Turn off mixer and gather dough into a ball.
Preheat oven to 350°F with the two racks set in the lower and upper thirds. Pour the sugar crystals onto small plate and grease a couple non-stick baking sheets with butter.
Use your hands to make small balls of dough of about 1 1/2- to 2-inches. Flatten slightly and cut 5 short lines around the edge of the disk with scissors, as if making petals, without cutting all the way into the center. Press down gently in the center to make a flower shape. Then flip over and press the top into the sugar crystals and then place right side up on the baking sheet. Repeat with all the dough balls, leaving about an inch of space between the cookies on the baking sheet.
Bake cookies for 30 to 32 minutes, until cooked and barely beginning to brown around the edges. Remove from the oven and let them cool on a cooling rack.
Once cool, make sandwiches with a couple teaspoons of cajeta or dulce de leche in between of two cookies with the sugar side facing outwards.