Preheat your oven to 350 degrees Fahrenheit. Line an 18x13 inch baking sheet with parchment paper.

In a large mixing bowl, cream together the butter, oil, granulated sugar, and powdered sugar.

Add the milk and eggs, and mix until combined.

In a separate bowl, whisk together the baking powder, cream of tartar, salt, and all purpose flour.

Slowly add the dry ingredients to the first bowl, mixing until just combined. Be careful not to overmix.

Spread the dough evenly onto the prepared baking sheet.

Bake at 350 for 13 to 15 minutes. The edges will be golden brown and the middle will still be pale, but a toothpick inserted in the center should not come away with any raw batter.

Allow the cookie bars to cool.

For the frosting, in a large mixing bowl, cream together the butter and sour cream.

Add the vanilla and mix.

Add the powdered sugar and a generous amount of orange gel food coloring, and mix until well combined.

Add milk, as needed for the perfect consistency. The frosting should be pourable. If needed, add additional orange color.

Pour the frosting over the sugar cookie bars and smooth it out evenly.

Top with Halloween sprinkles.

Allow the bars to chill in the refrigerator until ready to serve.

Cut into squares and enjoy.