Preheat the oven to 350°F and set aside an ungreased 9-inch tube pan.

Sift the cake flour 4–5 times into a bowl to aerate it.

Add 3/4 cup of sugar and rainbow sprinkles to the flour and mix with a fork.

In a stand mixer, beat egg whites with the cream of tartar, vanilla, and salt until frothy.

Gradually add the remaining 3/4 cup sugar and beat until stiff, glossy peaks form.

Gently fold the flour-sprinkle mixture into the meringue in three additions.

Transfer the batter to the tube pan and smooth the top. Bake for 45–50 minutes.

Invert the pan immediately and cool upside down until completely cool.

Whisk the egg whites and sugar in a heatproof bowl over simmering water until warm and the sugar has dissolved.

Remove from heat and whip the mixture until fluffy and glossy. Stir in the vanilla and salt.

Frost the cooled cake generously and optionally toast the frosting.

Slice the cake with a serrated knife and serve on delicate plates with berries or tea.

Finish with a scattering of extra sprinkles for visual delight.