Preheat the oven to 350 degrees F and line a 9x13 inch pan or line with parchment paper. I like to leave a bit of an overhang of parchment paper to make it easier to lift out the finished bars.
Separate the white chocolate so you have 6 ounces for the bars. Coarsely chop those 6 ounces.
In a microwave-safe bowl, combine the chunks of butter and 6 ounces of chopped white chocolate (1 and 1/2 bars). Place in the microwave and microwave in bursts of 20 seconds, stirring in between each burst for 15 seconds until melted and completely smooth.
Whisk in the white sugar and brown sugar and stir until smooth. Add in the eggs and vanilla extract an again, whisk until smooth. Add in the sea salt, cinnamon, and cream of tartar. Stir until combined. Add in the flour and stir until just combined. You don't want to overmix the batter. Chop the remaining 2 ounces of white chocolate and gently stir into the batter along with the 1/2 cup white chocolate chips if desired. Transfer the batter to your prepared pan.
In a small bowl stir together the sugar and cinnamon. Sprinkle this mixture evenly over the batter.
Bake for 30-35 minutes. Test the bars by inserting a toothpick into the center. If it comes out wet the bars need a few more minutes. If it comes out dry (with a few crumbs), they're done! (I like these on the softer/more chewy side so I take them out around 30 minutes. Leave them in longer for firmer bars). Remove and allow to completely cool before cutting into bars.