• In a large mixing bowl or stand mixer, beat to combine butter, oil, powdered sugar, brown sugar, water and vanilla extract until smooth. Add eggs and mix until just combined.
  1 cup salted butter, ¾ cup vegetable oil, 1¼ cups light brown sugar, 1 cup powdered sugar, 2 teaspoons vanilla extract, 2 tablespoons water, 2 large eggs
• In a medium mixing bowl, whisk together flour, salt, baking soda and cream of tartar. Gradually blend dry ingredients into wet ingredients on low.
  6 cups all-purpose flour, 1 teaspoon salt, ½ teaspoon baking soda, ½ teaspoon cream of tartar
• Cover and refrigerate dough 30 minutes. Preheat oven to 350°F and line 2 large baking sheets with parchment paper.
• Drop large (golf ball size) dough balls and press with a small greased glass to flatten and use the back of a spoon (or measuring spoon) to make a pool for the icing.
  
• Bake 8 minutes in the center of the oven until just set. Allow to set 1 minute and transfer to a cooling rack and allow to cool completely.

Icing

• In a medium mixing bowl or stand mixer fitted with the whisk attachment, cream butter, milk, sugar, brown sugar, vanilla extract and salt until light, fluffy and creamy, about 4 minutes. Optionally, you can take the extra step to melt icing in a measuring cup to pour into cookie pool, or use a teaspoon to fill.
  2 tablespoons butter, 3 tablespoons milk, ½ cup light brown sugar, 1 cup powdered sugar, ¼ teaspoon salt, 1 teaspoon vanilla extract