Preheat the oven to 350°F (175°C) and prepare a muffin tin with butter or parchment paper.

Cream together the softened butter and granulated sugar until light and fluffy, about 3-5 minutes. Gradually mix in oat flour and a pinch of salt until the dough resembles coarse crumbs.

Press portions of dough into the muffin tin, forming crusts about ¼ inch thick.

In a saucepan, combine chopped rhubarb, sugar, and lemon juice. Simmer for 5-7 minutes until rhubarb softens. Let cool slightly.

Whisk egg whites until frothy, then mix in sweetened shredded coconut, sugar, vanilla extract, and a pinch of salt.

Spoon the cooled rhubarb filling into each crust, then dollop the coconut mixture on top.

Bake for 20-25 minutes until the coconut topping is golden brown.

Allow the tarts to cool in the tin for at least 30 minutes before removing.