Add flour, sugar, cornstarch and salt to a large mixing bowl and whisk well. Add in butter and vanilla and use your hands to incorporate into the flour mixture until the dough starts to come together.
Transfer dough to the parchment-lined pan and press into an even layer. Use a fork to poke a few holes in the shortbread. Bake for 32-35 minutes.
While the shortbread is baking, add the condensed milk, butter and brown sugar to a medium saucepan over medium heat. Stir until melted.
Attach a candy thermometer to the side of the pan, then slowly heat the caramel mixture to 240˚-245˚F (soft ball stage), whisking occasionally to prevent burning. It can take up to 20 minutes to reach the temperature, so be patient.
Once it reaches the correct temperature, remove from heat and whisk in vanilla extract. Let cool for a few minutes, then pour the caramel over the shortbread into an even layer.
Add chocolate and coconut oil to a heat safe bowl. Microwave in 30 second intervals, stirring well between each interval, until fully melted.
Optional: to create the feathered pattern, melt 2-3 ounces of vegan white chocolate and transfer to a piping bag. Pipe horizontal lines across the chocolate, then drag a toothpick in vertical lines (perpendicular to the piped lines) to create the pattern. You must do this right away after adding the chocolate layer, otherwise it will set.