1. To make the sponge, place all ingredients in a bowl and mix well with an electric mixer until combined.
2. Place the mixture in a greased baking tin/tray and bake at 180°C for approximately 30 minutes until golden.
3. Turn out onto a wire rack to cool.
4. Make the meringue by placing the aquafaba and cream of tartar in the bowl of a stand mixer. Start mixing at a slow speed until the mixture is foamy. Gradually increase the speed until the mixture becomes white and glossy and stiff peaks begin to form.
5. Add the sugar while mixing at a fast speed, then add the vanilla extract. Keep whipping until fluffy and stiff peaks form.
6. To assemble the alaska, cut the cooked sponge into 1/2cm thick slices, then cut out 4 x 5cm circles.
7. Top the sponges with a scoop of the mango ice cream, trying to keep it round.
8. Place the meringue in a piping bag, then pipe the meringue over the ice cream, ensuring it is completely covered.
9. Place the alaskas in the freezer overnight.
10. For the orange syrup, place the sugar in a pan with the orange juice and lemongrass and simmer for 5 minutes to form a light syrup, then strain through a fine strainer and chill in the fridge until ready to use.
11. To serve, remove the baked Alaska from the freezer and, using a blow torch (or in a very hot 220°C oven), glaze the meringue until golden all over.
12. Place the Alaskas onto serving plates, garnish with some orange segments and mango cubes and pour over a little orange syrup.