Preheat the oven to 350°F (175°C). In a large mixing bowl, combine the whole wheat flour, old-fashioned oats, ground cinnamon, and salt. Add the grated cold butter and toss to coat, ensuring the butter is evenly distributed.

In a separate bowl, whisk together the egg yolks, Greek yogurt, honey, and vanilla extract. Gradually incorporate this mixture into the dry ingredients, stirring until a dough begins to form. If the dough appears too dry, add one tablespoon of cold water at a time until it comes together.

Lightly flour a clean surface and roll out the dough to a thickness of ⅛ inch. Using a sharp knife or pastry cutter, cut the dough into 4-inch by 4.5-inch rectangles.

Tip: If the dough is too soft to handle, refrigerate it for 10-15 minutes before rolling it out.

Spoon approximately one tablespoon of jam into the center of each rectangle, leaving a ¼-inch border along the edges.

Lightly brush the edges with buttermilk, then fold one side over the filling, pressing the edges together to seal. Flip each bar seam-side down on a parchment-lined baking sheet to secure the closure.

Tip: Gently press along the edges to prevent the jam from leaking while baking.

Transfer the assembled bars to the freezer for 10-15 minutes to help them retain their shape.

Once chilled, brush the tops lightly with buttermilk and bake for 15-20 minutes, or until the tops are lightly golden. Remove from the oven and allow the bars to cool for 5-10 minutes before transferring them to a container for storage.

Tip: Avoid overbaking to keep the bars soft and chewy.