Add the butter, granulated sugar, and brown sugar to the bowl of a stand mixer fitted with a paddle attachment. Beat on high speed for one minute, to ensure that the mixture is smooth and creamy. Add in the eggs, and beat together for 30 seconds.

Next, add in the powdered sugar, cornstarch, vanilla bean paste, and salt. Beat together for 30 seconds, or until fully incorporated. Lastly, add in the flour. Beat to incorporate.

Wrap the dough in plastic wrap, and chill in the refrigerator for 1 hour.

Once the dough has chilled, unwrap the plastic wrap, and place the dough onto a lightly floured surface. Preheat the oven to 350°F.  Line 1 - 2 large baking sheets with parchment paper, and set aside.

Use a rolling pin to roll the dough out into a large sheet, about ¼ inch thick. Use a 1 ½ to 2 inch round cookie cutter to cut out as many cookies from the dough as possible. Place the cookies on the prepared baking sheets. Combine the remaining scraps of dough into a ball, roll it out into a ¼ inch thick sheet, and cut more cookies. Repeat these steps until all of the dough is used.

Place 32 heart candies into the center of 32 of the cookies. Press the candy hearts into the dough so that they stick. You should have 32 cookies with candies, and 32 cookies without candies. 

Bake the cookies for 12 minutes. They will be slightly browned around the edges, but still soft to the touch. Remove the cookies from the oven, and place on cooling racks to cool completely.

Once the cookies are cool, make the frosting. Add all of the frosting ingredients to the bowl of a stand mixer fitted with the paddle attachment. Beat together on high speed for 2 minutes, or until a smooth and fluffy frosting has formed.

Spoon the frosting into a piping bag fitted with a star piping tip. Start with the plain cookies, without candy hearts. Turn each one upside down, and pipe frosting onto the bottom. Top with the candy heart cookies, heart-side up. Gently press the cookies together so that they stick together and make sandwiches.

Serve immediately. You may also store the cookies in airtight containers at room temperature for up to three days, or in the refrigerator for up to 5 days.