Preheat the oven to 220°C/gas mark 7 and line a baking sheet with baking parchment. Remove the ice cream from the freezer and leave to stand at room temperature to soften slightly. Cut 1 round from each of the brownies, using a 7cm round biscuit cutter and put them on the baking sheet. Sprinkle with the liqueur, if using.
Put the egg whites in a clean bowl and whisk, using an electric mixer, until stiff peaks form. Whisk in the caster sugar in a thin stream and continue whisking until stiff and glossy. Put 1 scoop of ice cream on each of the brownie circles.
Working quickly, spoon the meringue over the ice cream and use a palette knife to spread it all the way down the sides of the Alaskas, making sure the ice cream and brownies are completely covered. Make a few peaks in the meringue, using the tip of the knife. Bake for 6–7 minutes until the meringue is browned. Serve immediately, dusted with a little cocoa powder.