Preheat your oven to 350°F (175°C) and prepare a muffin tin with parchment liners.

In a mixing bowl, cream together the butter and sugar until light and fluffy, then mix in the flour and a pinch of salt.

Press the dough into the bottoms of each muffin tin cup for a sturdy tart base.

Cook the rhubarb with sugar and lemon juice in a saucepan over medium heat for 5-7 minutes until soft.

In a bowl, whisk the egg whites until frothy, then mix with sugar, vanilla, salt, and coconut until combined.

Spoon the cooled rhubarb mixture over the crusts, then top with the coconut mixture, pressing down gently.

Bake for 20-25 minutes until the tops are golden brown, then cool in the tin before removing.