Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.

Cream together the unsalted butter and granulated sugar until light and fluffy, then add egg yolks and vanilla, mixing until smooth.

Whisk together all-purpose flour and salt in a separate bowl and gradually add this to the creamed mixture until just combined.

Wrap the dough in plastic wrap and refrigerate for at least 30 minutes.

Scoop tablespoon-sized portions of dough, roll them into balls, and coat in a mixture of cinnamon and granulated sugar.

Place the sugar-coated balls on the prepared sheets, pressing an indentation in the center of each with your thumb, then fill with apple butter.

Bake for 12-14 minutes, until the edges are light golden brown, then cool on wire racks.