Beat softened butter until it's creamy.
Add sugar and mix well together.
The mixture should look light and fluffy.
This takes about two minutes of mixing.
Add the egg yolk to the mixture.
Pour in the vanilla extract too.
Mix everything until well combined.
The batter should be smooth now.
Mix flour and salt in another bowl.
Add this slowly to the butter mixture.
Stir until dough forms together.
Don't overmix the cookie dough.
Wrap the dough in plastic wrap.
Put it in the fridge for 30 minutes.
Chilled dough is easier to work with.
This step is very important.
Roll dough on a floured surface.
Make it about ¼ inch thick.
Cut out heart shapes with your cutter.
Cut small circles from half the hearts.
These will be the cookie tops.
Place cookies on the baking sheet.
Bake at 350°F for 10-12 minutes.
They should be lightly golden around edges.
Don't overbake them at all.
Let cookies cool completely on the rack.
Spread jam on the whole heart cookies.
Place the cut-out hearts on top.
Dust with powdered sugar to finish.