Preheat the oven to 200 degrees Celsius/ 400 degrees Fahrenheit (fan oven 180 degrees Celsius/ 360 degrees Fahrenheit). Line two baking trays with baking paper.
Dough: Mix the sugar, vanilla sugar or extract, cornstarch, and flour. Chop the butter into small pieces and add them to the mixture. Add the egg. Knead the dough with the hand-held mixer. Turn the dough onto the working surface and knead shortly to form a disc of dough.
Roll the dough on the lightly floured working surface and cut out the cookies with a heart cutter. I used the largest heart cutter. Next, knead the dough scraps again, roll them and cut out more cookies. Continue in this manner until all the dough is used. I had about 45 large heart-shaped cookies, but you could have more if you use a smaller cutter.
Bake: Place the cookies on the prepared baking tray and bake for about 12 minutes or until lightly golden. Keep an eye on them, the baking time depends on the size of the cookies as well, smaller hearts will need a shorter baking time.
Let cool for about 5 minutes on the trays, then transfer the cookies to wire racks to cool completely. Only add the icing when completely cold.
Icing: Mix the icing sugar with 7 tablespoons lemon juice until smooth, thick and glossy. Place about a quarter of this mixture into a piping bag fitted with a very small round tip. Set the piping bag aside.
Add 1 tablespoon lemon juice to the remaining icing in the bowl. Add only a few drops of lemon juice more if necessary; the mixture should be pourable but not thin. Mix well and divide the icing between two small bowls.
Color the sugar icing in one bowl in pink and the other one in turquoise. Place each mixture into piping bags.
Outline each cookie with some of the stiffer white icing. Let this icing set for about 5 minutes.
Pipe some of the colored icing onto the cookie, enough to cover the surface. Use a toothpick to properly spread the icing evenly inside the white icing frame.
Make dots: Place the cookies on the working surface in the order you flooded them. After flooding about 4 cookies or so, you can start making the dots on the first cookie you've flooded. The flooding should be just a little bit set at this point, but still wet enough to allow the dots to sink nicely into it.Pipe the dots with the white icing. Use a clean toothpick to smooth the dots, if necessary.
Let icing set: Place the cookies on baking trays lined with baking paper and place them in the oven. Set the oven at its lowest setting and leave the cookies in the oven for about 20-30 minutes to allow the icing to set hard.
This is my usual, fool-proof sugar cookie recipe. A whole batch makes lots of cookies, between 40 and 60, depending on the size of the cutters. If you want to make only half of the batch, halve the quantities, including the egg. To split an egg properly, beat the egg well in a small cup, measure it and only add half of the liquid egg to the dough.