In the bowl of a stand mixer fitted with the paddle attachment, add the butter, sugar, and powdered sugar. Beat together on high speed for 1 - 2 minutes, or until smooth and fluffy.

Next, add in the egg and egg yolk, and beat together until combined. Add in the flour, then teh salt, baking powder, and baking soda. Beat together for 1 minute to ensure that the flour is fully incorporated and smooth. If the mixture is too thick, add in the milk (although this will most likely be unnecessary.

Wrap the dough in plastic wrap, and place it into the fridge to chill for one hour.

Once the dough has chilled, preheat the oven to 350°F. Line a large baking sheet with parchment paper, and set aside.

Unwrap the dough, and place it on a lightly floured work surface. Use a rolling pin to roll out teh dough into a sheet about ¼ inch thick. Use a large, 4 inch, heart-shaped cookie cutter to cut as many hearts as possible. Then, use a 1 - 2 inch cookie cutter to cut out a small, heart-shaped window in each cookie. Carefully place the large cookie hearts onto parchment-lined baking sheets, leaving about 1 inch between each cookie. 

Roll the excess dough into a ball, and then roll it out again into a ¼ inch thick sheet. Cut out more cookies until all of the dough is used.

Place two, unwrapped, red Jolly Rancher candies into the center of each heart cookie. Bake the cookies for 12 minutes.

Once baked, remove the cookies from the oven, and place on a cooling rack to cool completely. 

When cool, gently remove the cookies from the parchment paper. Serve immediately, or store in an airtight container for up to one week.