Preheat the oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper on all sides.
In a stand mixer fitted with the paddle attachment, combine the cold, cubed butter, sugar, vanilla extract (if using), flour, and salt. Beat on medium speed until the dough starts to come together, about 2 minutes. The dough will be crumbly.
Press about two-thirds of the dough into the prepared pan in an even layer. Spread the jam evenly over the dough, leaving a small gap around the edges.
Crumble the remaining dough over the jam, leaving some spots of jam visible.
Bake for 25-30 minutes or until the top is golden brown.
Allow the bars to cool completely before slicing into squares. Serve and enjoy!