Make the peanut butter cookie crust

1. Beat the butter and the sugars, baking soda, cinnamon, and salt until smooth in a stand mixer or with a hand mixer. 
2. Add in the peanut butter, egg, and vanilla and beat until smooth.
3. Add in the flour and oats and mix just until the flour disappears.

Press the crust into the pan

1. Scoop the peanut butter cookie dough in the middle of a parchment

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   -lined sheet tray.
2. Use a large offset spatula

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   or a dough scraper

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   to press the dough into the bottom of the sheet tray.
3. Cover the dough with a piece of plastic wrap and bring the crust up the sides of the pan to leave room for the filling. Freeze the crust for 15 minutes.

Make the pastry cream

While the crust is baking, make a batch of vanilla pastry cream.

Bake the crust

1. Poke the crust all over with a fork. Bake the crust at 350 F for 18-20 minutes until it feels set and the edges are beginning to brown.
2. Press down on the crust with another sheet tray to flatten the crust slightly.

Assemble the pie

1. Place a bowl for whipping the cream in the freezer while you assemble the pie.
2. Slice enough bananas to cover the peanut butter crust.
3. Smooth the pastry cream over the layer of bananas.
4. Warm up the marshmallows in a small cup for about 30 seconds in the microwave–long enough for the marshmallows to expand but not harden.
5. Place the marshmallows, cream, powdered sugar and vanilla in the chilled bowl and whip everything until the cream holds its shape.

Finish the top of the pie

1. Smooth the whipped cream over the pastry cream. Add a few more sliced bananas over the top of the pie and sprinkle the whole thing with cinnamon. 
2. Cover the pie with an upside down sheet tray and place in the fridge to chill for at least 4 hours before serving.
3. To serve, cut the pie into rows of slices.  20 slices (5×4 grid) makes for good-sized portions.
4. Keep any leftovers covered in the fridge for up to 3 days.