Beat the butter, sugar, lemon zest, lemon juice and vanilla extract on medium-high speed for 2 minutes, or until pale and fluffy.
Add the flour and salt. Mix for a few seconds on low speed, just until combined.
Preheat the oven to 160°C (325 degrees F) and line a baking tray with parchment paper.
Take a tablespoon worth of dough and roll it into a ball. You should get about 26 balls.
Put the balls into the freezer for 15 minutes to let them firm up and reduce the spreading.
Place the balls on the prepared baking tray at least 4 cm apart.Bake the cookies for 10 minutes, take them out of the oven, and press your heart stamp cookie cutter (check the notes) in the middle.
Place them back into the oven, and bake for additional 12-14 minutes, depending on the size and how soft you want them. Cookies should still be pale. When they come out of the oven, you could press them again with a heart stamp to get a better and deeper shape.
Let the cookies cool completely, meanwhile make the ganache.