Brown the butter: Place butter in a saucepan over medium-low heat. Allow to melt, then continue to cook, occasionally stirring gently. The butter will begin to pop and fizz: When it stops, watch closely, occasionally giving a little stir with a spatula. Shortly after the popping stops, it will begin to brown: you will probably smell it! Little brown flecks will appear at the bottom. Stir a few more times until a lot of brown bits are appearing, then remove from the heat. When in doubt, remove it: you don't want it to burn.

Set the browned butter aside to cool.

Preheat the oven to 325 degrees Farenheit. Line two baking sheets with parchment paper and set aside.

When the butter is cool but still liquid, place in a large mixing bowl.

Stir in the sugar and the vanilla, until well combined.

Whisk together the flour and baking soda in a separate bowl, then gradually add this mixture to the butter mixture, mixing until well combined. You will have a very crumbly dough.

Add the yolk to the dough and mix well. You can use forks, a pastry cutter, or a handheld beater for this. Mix until the yolk is well dispersed and mixed in. The dough will still be very crumbly, but it should stick together when you press it between two fingers.

Use a spoon to shape the cookies: Press a spoonful of dough together, so that it forms a spoon/shell shaped cookie. Slide each onto the prepared baking sheets.

Bake for 7-11 minutes (will vary based on the size of spoon that you used!) The cookies will appear puffy and very lightly browned.

Let the cookies cool on the baking sheets. They will be far too soft to handle while they are hot.

Once cool, turn half the cookies so that the flat side is up. Spread a little jam on each, then top with another cookie.

Pour some granulated sugar into a small bowl, and gently spoon each cookie through the sugar, turning to coat all sides.

Remove the sugared cookies to a cooling rack.

Enjoy! These keep wonderfully for a few days at room temperature, in a sealed bag or container. Some even prefer them after a day or two, when some of the jam has leached into the cookies. You can also freeze them for a few months!