Preheat oven to 350°F. Grease an 10x15 baking sheet/jelly roll pan*.
Using a hand mixer or stand mixer fitted with the paddle attachment, beat butter, granulated sugar and oil on medium speed until smooth and combined. Add eggs, sour cream, and vanilla and beat until combined, scraping the sides of the bowl as necessary.
In a separate bowl, combine flour, cream of tartar, baking soda, and salt.
With the mixer on low, add the the dry ingredients to the wet ingredients and mix until combined, scraping the sides of the bowl as necessary. Transfer the dough into the greased pan and spread into an even layer. In a small bowl, combine the sugar and cinnamon for the topping. Sprinkle evenly over the dough (after sprinkling, I like to lightly press the cinnamon sugar onto the dough with my hands). Bake for around 20 minutes*, until the center no longer jiggles and the edges are beginning to turn a light golden brown. Be careful not to over bake. Place the pan on a wire rack to cool.
Cinnamon icing:
Whisk all ingredients until smooth and combined. Adjust the consistency by adding more powdered sugar (to thicken) or more half and half (to thin). Drizzle the icing over the cooled bars.
Notes
*I prefer baking these bars in an 10x15 pan. If you don't have that size you can use a 9x13; just note that the bars will be thicker. The bake time for the 10x15 pan is around 20 minutes and the bake time for the 9x13 is 25-30 minutes.