Preheat your oven to 350°F (175°C) and prepare a muffin tin by greasing each cup with butter or lining with parchment paper.

Cream together ½ cup of softened unsalted butter and ½ cup of granulated sugar in a mixing bowl until light and fluffy, about 3–4 minutes.

Mix in 1 cup of all-purpose flour and a pinch of salt to form a crumbly dough.

Press the dough evenly into the prepared muffin tin to form the crust, slightly coming up the sides.

Combine 1 cup of chopped fresh rhubarb, ¼ cup of sugar, and 1 tablespoon of lemon juice in a saucepan and cook over medium heat for 5–7 minutes until soft.

Whisk 2 egg whites until frothy, and fold in 1½ cups of sweetened shredded coconut, ¼ cup of sugar, 1 teaspoon of vanilla extract, and a pinch of salt.

Spoon the rhubarb filling into each crust, then top generously with the coconut mixture.

Bake for 20–25 minutes until the tops are golden brown and the coconut is lightly toasted.

Let the tarts cool in the tin for about 10 minutes before transferring to a wire rack to cool completely.