Line the pan. Line a 9x13-inch pan with parchment, leaving overhang on the long sides for easy lifting later. Lightly grease the parchment to prevent sticking.

Make the brownie batter. Melt the butter in a saucepan over low heat. Off the heat, whisk in sugar, cocoa, vanilla, and salt until glossy. Whisk in the eggs one at a time. Fold in the flour just until combined. Stir in chocolate chips if using.

Bake the brownie base. Spread batter evenly in the lined pan. Bake at 350°F (175°C) for 18–22 minutes, until the top looks set and a toothpick comes out with moist crumbs. Do not overbake; you want a chewy base.

Cool completely. Let the brownies cool in the pan on a rack. Slide the pan into the fridge for 30 minutes to firm up before layering.

Soften the ice cream. Set the ice cream on the counter for 10–15 minutes until it’s scoopable but not melting. You want it spreadable without becoming soupy.

Spread the ice cream layer. Dollop softened ice cream over the chilled brownie. Use an offset spatula to spread it into an even layer. Press out air pockets and smooth the top. If you’re adding swirls or mix-ins, fold them into the ice cream first or gently ripple them after spreading.

Freeze to set. Cover the pan and freeze for at least 2 hours, or until the ice cream is firm to the touch. Longer is fine; overnight is ideal for clean slices.

Make the chocolate topping. Melt chocolate chips with oil and a pinch of salt in short bursts in the microwave (stir every 20–30 seconds) or over a double boiler. Stir until smooth and glossy.

Top and set. Pour the warm (not hot) chocolate over the ice cream layer. Tilt the pan to spread it into a thin, even sheet. Sprinkle nuts or flaky salt if you like. Freeze again for 30–45 minutes, until the top is set but not rock hard.

Slice and serve. Lift the whole slab out using the parchment. Warm a sharp knife under hot water, wipe dry, and cut into bars or squares. Work quickly and return extras to the freezer.