Preheat the oven to 350°F & line two baking sheets with parchment paper.
Cream together the softened butter, sugar & salt in the bowl of a stand mixer with paddle attachment for 3 to 4 minutes, until the mixture is light, pale & fluffy.
Add the egg, vanilla extract & nutmeg & mix on low until combined.
Once combined, gradually add the flour until the dough has come together.
Pour the dough onto a lightly floured surface & form it into a ball. Chop the ball in half, set one half to the side & roll out the dough until it's just under ¼ inch thick.
Use the round cutter to cut out 10 circles, re-rolling & cutting as needed. Transfer each cookie onto one of the baking sheets, then chill the sheet in the fridge for about 5 minutes.
Bake the first sheet for 9 to 11 minues, until the cookies START to brown around the edges.
Meanwhile, roll out the second half of dough & repeat untill the next ten cookies are laid out on the second baking sheet. Use the heart-shaped mini cutter turned upside down to cut out a planchette shape in each. Stick the second sheet in the fridge to chill for five minutes, then bake it same as the first.
Let the cookies initially cool on the baking sheet. After five or so minutes, transfer each to a wire rack to fully cool.
Once completely cool, spoon about one tablespoon of jam onto each base cookie & sandwich a planchette cookie on top.