Preheat your oven to 350°F (175°C) and lightly grease a muffin tin or line it with parchment liners.
Cream together the softened unsalted butter and granulated sugar until light and fluffy, about 3 minutes. Gradually mix in the flour and a pinch of salt until crumbly.
Press the dough into the bottom of each muffin cup to form crust bases.
In a saucepan, combine chopped rhubarb, granulated sugar, and lemon juice. Simmer for 5–7 minutes until soft. Let cool.
Whisk the egg whites until frothy, then mix in sugar, vanilla, salt, and shredded coconut until combined.
Spoon rhubarb filling over crusts, then add the coconut topping, pressing down gently.
Bake for 20–25 minutes until golden brown on top. Let cool in the tin.
Carefully lift each tart out to serve.
Notes
Allow the tarts to cool completely in the tin for easier removal. For best results, use fresh rhubarb and adjust sugar to taste.