• Preheat oven to 350 F. Line a 9-by-13-inch cake pan with parchment paper, allowing ends to hang over by 2 inches. Coat with butter or nonstick spray. Set aside.

Cookie Base

• Stir together the flour, cream of tartar, baking soda, and salt; set aside.
• Cream the butter and sugar until light and fluffy, about 3 minutes.
• Add the egg, cream, and milk, beat until combined.
• Stir in flour mixture until just incorporated.
• Dollop the dough into the prepared pan, ensuring you get some near the edges. Use a butter knife even out the dough dollops into a flat base surface, then set aside.

Gooey Layer:

• Whisk together the corn syrup, cream, and vanilla in a bowl and set aside.
• Cream butter, sugar, and salt until fluffy.
• Add the egg and beat until combined.
• Add the flour and corn syrup mixture alternately, mixing each time until all ingredients are incorporated.
• Dollop over the cookie base and spread into an even layer.

Assembly

• Mix together the sugar and cinnamon; sprinkle evenly over the gooey layer.
• Bake until edges have set but center is still jiggly and gooey, about 25-35 minutes, depending on your oven. (They took 33 minutes in my old oven, 25 minutes in my new oven.)
  
• Let cool completely, about 2 hours.
• Lift from pan using the parchment; cut into 1-inch squares.
• Store at room temperature for up to 3 days.