Heat the oven to 180 C / 350 F. Butter a 24 cm / 9.5-inch ring pan, dust it with 1 tbsp flour and tap out the excess. Whisk the flour, baking powder and 1/4 tsp salt together in a bowl.

Beat the butter and granulated sugar with a mixer for 3 minutes until pale and fluffy. Add the eggs one at a time, beating well after each, then mix in the lemon zest.

Add the flour mixture in three additions, alternating with the milk and Alchermes mixed together. Stir on low speed just until no dry streaks remain. The batter will be thick but pourable and pink.

Scrape the batter into the pan and level the top. Bake on the middle rack for 35 to 40 minutes, until a skewer inserted near the center comes out clean and the top springs back.

Cool the cake in the pan for 15 minutes, then turn it out onto a wire rack. Let it cool for 30 minutes more, until it feels fully room temperature.

Lower the oven to 150 C / 300 F. Line a 33 x 46 cm / 13 x 18-inch baking sheet with parchment and set the cooled cake on it.

Whisk the egg whites with the pinch of salt and lemon juice on medium speed until soft peaks form. Add the caster sugar 1 tbsp at a time, then beat on high for 5 to 6 minutes until the meringue is stiff, glossy and doesn't feel gritty between your fingers.

Spoon the meringue over the top and upper sides of the cake in thick swirls. Scatter the sprinkles over it right away, while the surface is still wet.

Bake for 10 to 12 minutes, until the meringue is set, dry to the touch and pale ivory. Cool on the pan for 10 minutes before moving it to a serving plate.