Preheat the oven to 350 degrees F. Spray a 14-inch x 4-1/2-inch rectangular tart pan with non-stick cooking spray and line the bottom with parchment.

In the bowl of a food processor, pulse 1-3/4 cups pretzels until they have the consistency of flour. Combine 1/2 cup pretzel flour, all-purpose flour and baking powder in a medium bowl.

Discard the leftover pretzel flour. Pulse the remaining whole pretzels until they are roughly chopped.

Using an electric or stand mixer, cream the butter and sugar until light and fluffy. Mix in the egg yolk and then the dry ingredients until just combined. Fold in 1/2 cup chopped pretzels.

Spread the dough into the prepared tart pan and press 1/4 cup chopped pretzels on top. Bake for 25-30 minutes until the shortbread is light brown and firm to the touch. Cool completely on a wire rack before removing from the pan and cutting into 1-inch wide rectangles with a serrated knife.

Store in an airtight container.