Prep the pan: Line an 8x8-inch square pan with parchment, leaving overhang for easy lifting. Lightly grease the sides.
Make the crust: Stir graham crumbs, melted butter, brown sugar, and salt until the mixture looks like damp sand.
Press firmly into the pan in an even layer. Chill for 10 minutes.
Mix the peanut butter layer: Beat peanut butter, cream cheese, powdered sugar, vanilla, and milk until smooth and spreadable. Adjust with a splash more milk if it’s too thick.
Spread the peanut butter: Spoon onto the chilled crust and smooth to the edges.
Chill while you prepare bananas.
Add bananas: Toss banana slices with a little lemon juice if using. Lay them in a single, snug layer over the peanut butter.
Make the chocolate topping: Melt chocolate chips with coconut oil in the microwave in 20–30 second bursts, stirring until glossy.
Top and chill: Pour chocolate over the bananas and tilt the pan to coat evenly. Sprinkle flaky sea salt if you like.
Chill uncovered for 15 minutes, then cover and chill at least 2 hours, until set.
Slice and serve: Lift out using the parchment. Warm a sharp knife under hot water, wipe dry, and cut into squares.