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

In a mixing bowl, whisk together the flour, baking soda, cinnamon, and salt.

In another bowl, cream the softened butter with dark brown sugar and granulated sugar until light and fluffy.

Add in the egg and vanilla extract and mix thoroughly until fully incorporated.

Gradually fold the dry ingredients into the wet mixture and stir in the oats.

Roll the dough into balls about 1 inch in diameter and place them on the baking sheets, spaced 2 inches apart.

Gently press down in the center of each cookie ball to create a thumbprint.

Fill each indentation with about ½ teaspoon of jam.

Bake for 12-14 minutes until edges are lightly golden and centers are set but soft.

Allow to cool on the baking sheets for 5 minutes before transferring to a wire rack to cool completely.