Preheat the oven to 400°
Prepare a baking sheet by lining it with parchment paper. Take a 10" springform pan apart. Remove the bottom plate and reclip the top piece. Put this on the parchment and set aside
Add the flour, oat bran, confectioner's sugar, cornstarch and salt to the bowl of a stand mixer with the paddle attached. Mix on low until combined.
With the mixer running toss the pieces of butter in, one by one. Continue mixing until the dough comes together - about 3 - 5 minutes.
Press the dough onto the prepared baking sheet using the springform pan top as a guide.
Be sure to press the dough evenly along the bottom and about 1/2" up the sides
Prick the dough with a fork all across the bottom
Bake in the preheated oven for 5 minutes then lower the temperature to 250° and bake for another 12 - 15 minutes until lightly browned.
Carefully remove from baking sheet to a wire rack to cool.
Remove the springform and cool completely.
Melt the chocolate and the coconut oil until smooth.
Pour over the top of the shortbread and carefully tilt the tart until the chocolate evenly covers the top.
Place the cookie tart in the freezer to help the chocolate set.
When the chocolate has hardened, pour the caramel sauce over top and tilt the tart to spread evenly.
Top with the toasted walnuts.