Step 1: Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving an overhang on the sides to easily lift the bars out later.
Step 2: In a large bowl, cream together the softened butter and granulated sugar until light and fluffy. Gradually add the flour and salt, mixing until just combined and a soft dough forms.
Step 3: Press the dough evenly into the bottom of the prepared baking pan. Bake for 20-25 minutes, or until the edges are lightly golden. Remove from oven and let cool slightly on a wire rack.
Step 4: While the crust is cooling, melt the white chocolate. You can do this in a microwave-safe bowl in 30-second intervals, stirring between each, or over a double boiler. Stir until smooth. If using, stir in the peppermint extract.
Step 5: Pour the melted white chocolate evenly over the warm shortbread crust.
Step 6: Immediately sprinkle the crushed peppermint candy over the white chocolate layer, pressing gently to ensure it adheres.
Step 7: Let the bars cool completely at room temperature, then transfer the pan to the refrigerator for at least 1 hour, or until the chocolate is firm.
Step 8: Once chilled, use the parchment paper overhang to lift the entire slab out of the pan. Place on a cutting board and cut into 16 squares or desired shapes. Serve and enjoy!