Prep the pan. Line an 8x8-inch (20x20 cm) baking pan with parchment, leaving overhang on two sides for easy lifting. Lightly grease the parchment.

Preheat the oven. Set to 350°F (175°C). This ensures the base bakes evenly from the start.

Cream the butter and sugar. In a mixing bowl, beat the butter and sugar until light and fluffy, 2–3 minutes. Add the egg and vanilla; mix until smooth.

Combine dry ingredients. In a separate bowl, whisk flour, baking powder, and salt. Add to the wet mixture and stir until just combined. Do not overmix.

Press and bake the base. Press the dough evenly into the prepared pan. Bake for 12–15 minutes, until the edges are set and lightly golden. The center should look slightly soft.

Add marshmallows. Immediately scatter the mini marshmallows over the hot crust. Return to the oven for 2–3 minutes, just until puffy and sticky, not browned.

Make the topping. While the marshmallows puff, melt the chocolate chips and peanut butter together in a microwave-safe bowl in 20–30 second bursts, stirring between each, until smooth. Stir in the oil and a pinch of salt if using.

Spread the chocolate layer. Gently tilt or spread the warm chocolate mixture over the marshmallows. If the marshmallows lift, use a light touch and small strokes. Sprinkle nuts or flaky salt if desired.

Cool and set. Let the pan cool at room temperature for 45–60 minutes, then transfer to the fridge for 20–30 minutes to firm. Don’t chill too long or cutting gets harder.

Slice and serve. Lift the slab using the parchment. Warm a sharp knife under hot water, wipe dry, and cut into squares. Serve slightly cool or at room temperature for peak gooeyness.