Preheat the oven to 350°F (175°C).

Cream together 1 cup of softened unsalted butter and 1 cup of granulated sugar until light and fluffy.

Beat in 1 large egg, along with 1 tablespoon of freshly grated lemon zest and 2 tablespoons of freshly squeezed lemon juice until well combined.

Whisk together 2 cups of all-purpose flour, 1 teaspoon of baking powder, and 1/2 teaspoon of salt in another bowl.

Gradually add the dry ingredients to the wet mixture, mixing until just combined.

Fold in 1 cup of diced rhubarb gently.

Drop tablespoon-sized portions of dough onto a baking sheet lined with parchment paper.

Bake in the preheated oven for 10-12 minutes or until the edges are lightly golden.

Remove from the oven and let cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.

Dust with 1/2 cup of powdered sugar before serving.