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

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

Press the mixture firmly into the bottom of each muffin tin cup to form a crust base.

In a saucepan, combine chopped rhubarb, sugar, and lemon juice. Cook over medium heat for 5-7 minutes until syrupy.

Remove from heat and let cool slightly before using.

In a clean bowl, whisk egg whites until frothy, then fold in shredded coconut, vanilla extract, and salt.

Spoon the cooled rhubarb mixture over each crust, then add the coconut topping on top, pressing down gently.

Bake in the preheated oven for 20-25 minutes until golden brown.

Allow to cool completely in the tin for about 30 minutes before serving.