Sift all the flour and 3/4 cup of the sugar into a large bowl using a fine mesh strainer. Sift another 2 times. Set aside.
Combine the egg whites, cream of tartar and salt in the bowl of a stand mixer with the whisk attachment fitted. Beat on medium until soft peaks form when the whisk is lifted up.
Turn the stand mixer back on at a medium speed. Gradually stream in the remaining 1 cup of sugar into the egg whites as they’re being whisked. Once it’s all been added, let the mixer run on high speed until stiff peaks form when the whisk is lifted up. Add the almond extract and vanilla extract and beat in until just combined.
Take 1/3 of this beaten egg white mixture and add to the bowl of the flour and sugar. Mix together to combine. Add this to the bowl of remaining beaten egg whites and fold together gently with a spatula until just combined, being careful not to knock too much air out of the egg whites while doing this. Finally, fold in the rainbow sprinkles with a few strokes – don’t overmix or the sprinkles will start to bleed into the batter and discolour it.
Spoon the batter into an ungreased, 10-inch diameter tube pan. Smooth out into an even layer.
Bake for 35-40 minutes until golden, the top of the cake should spring back when gently pressed with a fingertip. Invert the cake onto a wire rack and leave in its pan to cool completely, about 2 hours.
Once cooled, use a metal spatula or butter knife to loosen the cake from the edges of the pan. Tip out onto a wire rack.
Make the candy melt fireworks:
Tip the different coloured candy melts into separate, heatproof bowls. Set each one over a pan filled with 1 inch of simmering water on the stove. Let melt, stirring occasionally, until they’re fully melted and smooth.
Divide the colours into individual piping bags fitted with small round tips.
Pipe firework shapes of different colours, sizes and shapes onto a piece of parchment paper. Set aside to cool and set.
Make the frosting:
Place the cold heavy cream into a large bowl and beat with a whisk or electric beaters until it starts to thicken. Add the powdered sugar and briefly stir inches
Take 4 small bowls and add 1/4 cup of the whipped cream to each bowl. Add pink food colouring to one bowl, green to another, purple to another and yellow to the last small bowl. Stir through so you have one small bowl of each of the colours. Divide these into 4 separate piping bags fitted with a small round tip.
To the remaining whipped cream in the large bowl, add the blue food colouring and stir through.
It’s important not to over-stir the cream or it will start to separate. If it starts to look grainy you can pour a little un-whipped heavy cream into the bowl and gently stir that in which should help bring it back from separation.
Frost the cake:
Cover the cooled cake with the blue frosting, using an offset spatula to spread it over the top, sides and inner hole in a smooth layer.
Take your other colours of whipped cream frosting and pipe fireworks all over the top and sides of the cake.
Insert the candy melt fireworks you made earlier into the top of the cake.
Serve immediately or keep chilled, covered for up to 2 days.