Make the ganache first. In a bowl or tupperware, add the 1/2 cup cream. Microwave on high for about 1 to 1.5 minutes, or until the edges have started to bubble.
Immediately add the chocolate chips and let it sit for about 1 minute. Use a whisk to slowly combine the cream and chocolate. It should be smooth when you are done. Add a bit of salt and 1 tablespoon butter. Cover and put in the freezer for about an hour, or until it is cold enough to be hard to the touch.
Preheat your oven to 350 degrees F. Grease a 9x9 or 8x8 inch square pan, or line with foil (and grease) or parchment paper.
In a bowl, stir together melted butter, brown sugar and white sugar. Stir in the peanut butter, vanilla, eggs, and milk. Combine well.
In a separate bowl, whisk together flour, baking powder, and salt. Add to the wet ingredients and mix until just combined.
Spread the batter into the prepared pan. Take the ganache out of the freezer, and use a spoon to scoop it out and press into the batter. My scoops were probably 1-3 teaspoons each. It gets pretty melty pretty fast. By the end of adding mine it had pretty much returned to room temperature, and had swirled quite a bit. That's ok! Cover up the ganache with the batter as best you can.
Sprinkle 1-2 tablespoons white sugar over the top of the batter.
Bake at 350 for 28-29 minutes, or until the center has set and the top is golden. Let cool at least a few minutes before cutting.