Preheat the oven to 350°F and line a 9x13 inch pan with parchment paper. Set aside.
In a large bowl, use a hand mixer to beat together butter, brown sugar and granulated sugar together until combined.
1/3 cup unsalted butter, 3/4 cup brown sugar, 1/2 cup granulated sugar
Add in the peanut butter, eggs, and vanilla and beat to combine.
1/2 cup creamy peanut butter, 2 eggs, 1 teaspoon pure vanilla extract
Use a spatula to fold in the flour, baking powder, and salt.
1 cup all-purpose flour, 1 teaspoon baking powder, 1/2 teaspoon salt
Spread evenly into prepared pan and bake for 28-30 minutes, or until a toothpick inserted into the center comes out with a moist crumb.
Let cook in pan for 5 minutes and then lift out and let cool completely on a wire rack.
Fluff layer
Place the egg whites and cream of tartar in the bowl of a stand mixer fitted with a whisk attachment and whip the egg whites to soft peaks on medium speed.
2 large egg whites, 1/4 teaspoon cream of tartar
At the same time, place maple syrup and salt in a medium sauce pan and bring to a boil over high heat. Continue boiling until the mixture reaches 240°F on a thermometer.
1 1/4 cups pure maple syrup, 1/8 teaspoon salt
When the syrup reaches 240°F, reduce the mixer speed to low and slowly add vanilla. Increase the speed to medium-high and whip for about 7 minutes - the marshmallow crème is stiff and glossy.
2 teaspoons pure vanilla extract
Add the vanilla and whip an additional 2 minutes.
Frost cooled peanut butter bars, cut into 24 bars and enjoy!
Notes
Cover and store leftover bars at room temperature for up to a week.