In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and sugar for 2 minutes until light and fluffy. Scrape down the bowl with a rubber spatula.
Add the egg and vanilla and beat for another minute.
Mix together the flour, cornstarch and salt then add to the butter mixture in two batches, mixing well after each addition. Remember to scrape the bowl down with a spatula to ensure the ingredients are evenly incorporated into the dough.
Tip the dough onto a sheet of parchment paper then cover with another piece of parchment paper and roll into a rectangle using a rolling pin, approximately ⅛-inch (½cm) thick.
Place in the refrigerator and allow to chill for 30 minutes.
Once the dough has firmed up, use a heart-shaped cookie cutter and cut out cookies. Push the off-cuts back together before rolling out to get more cookies out of each batch. For a cute effect, I used a smaller cookie cutter to cut hearts out of the centers of half of the cookies for a peekaboo effect. This is optional.
Place the cookies on 1-2 baking sheet lined with parchment paper, spacing them out slightly and refrigerate for another 30 minutes.
Preheat the oven to 400°F/200°C.
Bake the cookies for 10-12 minutes until lightly golden around the edges.
Remove from the oven and allow to cool completely then add a teaspoon of jam to half of the cookies and sandwich with the remaining cookies.