Place bananas on a rimmed baking sheet, and sprinkle with brown sugar. Bake for 20 minutes. Let cool.
In a medium saucepan, combine ¾ cup milk and 2 tablespoons granulated sugar. Bring to a boil over medium heat, stirring until sugar is dissolved.
In a medium bowl, combine cornstarch and remaining 2 tablespoons granulated sugar. Whisk in remaining ¼ cup milk. Add yolks and vanilla, stirring until smooth.
Temper egg mixture by whisking in about one-third of hot milk mixture. Return egg mixture to remaining hot milk in saucepan. Cook, whisking vigorously, until mixture comes to a boil. Pour pastry cream into a bowl, and press a sheet of plastic wrap directly onto surface of cream. Refrigerate for at least 4 hours or overnight.
In the bowl of a stand mixer fitted with the paddle attachment, beat bananas at medium speed until broken up but still chunky. Add cream mixture and peanut butter, and beat until combined. If preparing a day ahead, cover with plastic wrap and refrigerate.
Recipe by Louisiana Cookin at https://www.louisianacookin.com/elvis-king-cake/