In a medium saucepan, combine the chopped rhubarb and 1/2 cup of sugar. Cook over medium heat, stirring occasionally, until the rhubarb is soft and breaks down, about 10 minutes. Remove from heat and let it cool slightly.

In a mixing bowl, whisk together the eggs, remaining 1/2 cup of sugar, vanilla extract, and salt until well combined. Gradually stir in the warm rhubarb mixture.

Return the mixture to the saucepan and cook over low heat, stirring constantly, until it thickens enough to coat the back of a spoon, about 10-15 minutes. Do not let it boil.

Remove from heat and stir in the softened butter and lemon juice until fully incorporated.

Strain the curd through a fine-mesh sieve into a bowl to remove any solids.

Cover the bowl with plastic wrap, pressing it directly against the surface of the curd to prevent a skin from forming. Refrigerate for at least 2 hours or until set.

Serve chilled as a filling for tarts, spread on toast, or as a topping for desserts.