Preheat the oven to 375°

Cream together the butter, oil, sugar, vanilla and salt in a large mixing bowl, just until the mixture is smooth.

Add the egg and mix just until incorporated.

Mix in the cornstarch, cocoa powder and 1 1/4 cups of flour. The dough should form into a ball and pull away from the sides of the bowl. Add more flour by the tablespoonful if necessary.

Roll the dough to a thickness of 3/8-inch, between two sheets of parchment paper.

Cut shapes using a cookie cutter.

Bake on parchment-lined baking sheets for 9 to 12 minutes, or until the cookies look dry on the surface, and feel firm around the edges and set in the middle.

Cool completely before decorating