Generously butter and 9 x 13 inch baking dish and preheat the oven to 350º

Cream together the butter and peanut butter.

Add the sugars and beat until fluffy.

Beat in the eggs, one at a time.

Beat in the vanilla.

Measure in the flour, salt and baking powder.

Beat on medium speed until well combined - about 30 seconds.

Beat in 3/4 cup of the milk chocolate chips, reserving the rest for the top.

Spread the batter evenly in the prepared pan, and bake at 350º for 28 to 30 minutes.* Cool completely.

While the bars are cooling, heat the heavy cream just to a simmer, remove from the heat and add the remaining chocolate chips.

Let the cream and chips sit for a couple of minutes, and then stir until the chips are completely melted and the mixture is very smooth.

Once both the bars and the ganache are cooled to room temperature, spread the ganache in an even layer, and then sprinkle lightly with coarse sea salt, like Maldon.

You can refrigerate the bars at this point to allow the ganache to harden, or just let them sit for a few hours at room temperature.

Cut into bars to serve. Store leftovers tightly sealed in a single layer - refrigerate after 2 days.