Start by creaming together the peanut butter, butter, brown sugar, and granulated sugar until smooth. Beat in the egg until fully incorporated. Then gently fold in the flour, baking soda, and salt until the dough comes together.
Use a small cookie scoop or your hands to form 1-inch balls of dough. Place them evenly on the prepared baking sheet, leaving some space for spreading. Press your thumb gently into the center of each cookie to create a small well.
Spoon your chosen jam into each thumbprint indentation. Be careful not to overfill, as the jam may spill during baking. This step gives the cookies their signature sweet center.
Bake in a preheated oven at 350°F (175°C) for 10–12 minutes. The cookies should be lightly golden at the edges but still soft in the middle. Let them cool slightly on the baking sheet before transferring to a wire rack.