Preheat oven to 325°F. Use butter to grease a 9-inch springform pan or 9-inch tart pan with removable bottom. Add flour, sugar, cocoa powder and salt to a food processor. Cover and pulse until combined.
Add butter and red food coloring and process until mixture comes together into a ball. It will take about 30-45 seconds.
Press mixture into the prepared pan into an even layer. Note: If your pan is not nonstick, line bottom with parchment paper.
Bake until center is set, 15-18 minutes. Let cool completely in the pan.
Remove sides from pan and use a knife to loosen the cookie from the bottom. Transfer to a cutting board and cut into 12 wedges.
Melt white chocolate in a heat proof bowl set over a pan of simmering water. Remove from heat when a few lumps remain, then stir until smooth.
Drizzle white chocolate over the wedges of shortbread. Let sit for 10 minutes until set.
Store cookies in an airtight container for a week or more.