Preheat your oven to 375°F (190°C). Grease and flour a standard 12-cup muffin tin, or line with paper liners.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.

In a large bowl, cream together the softened butter and granulated sugar using an electric mixer until light and fluffy, about 3 minutes.

Beat in the eggs one at a time until fully incorporated. Mix in the vanilla extract.

Alternately add the dry ingredients mixture and the buttermilk to the wet ingredients, beginning and ending with the dry ingredients. Mix only until just combined; do not overmix.

Gently fold in the chopped rhubarb until evenly distributed throughout the batter.

Spoon the batter evenly into the prepared muffin cups, filling each about two-thirds full. Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean.

While the cakes cool slightly (about 10 minutes in the tin), prepare the glaze. Whisk together the powdered sugar and lemon juice until smooth. Adjust thickness as needed by adding a few extra drops of juice or tablespoons of sugar.

Transfer the cakes to a wire rack to cool completely. Drizzle generously with the lemon glaze before serving.