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

Cream together the unsalted butter and granulated sugar until fluffy and pale. Gradually mix in the all-purpose flour and salt until the dough is crumbly. Press the mixture into the muffin cups.

In a saucepan, combine the chopped fresh rhubarb, sugar, and lemon juice. Cook over medium heat until the rhubarb softens, then remove from heat and allow to cool.

Whisk egg whites until frothy and soft peaks form. Gently fold in the shredded coconut, sugar, vanilla extract, and a pinch of salt.

Spoon the rhubarb filling onto each crust. Then dollop the coconut mixture over the filling, pressing it down gently.

Bake for 20-25 minutes, or until the topping is golden brown. Allow tarts to cool in the tin.

After cooling for 10-15 minutes in the tin, transfer the tarts to a wire rack to cool completely before serving.