Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x5 inch loaf pan, or line with parchment paper, leaving an overhang on the long sides for easy removal.
Step 2: In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Set aside.
Step 3: In a large bowl, using an electric mixer, cream together the softened butter and granulated sugar until light and fluffy, about 2-3 minutes.
Step 4: Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract and lemon zest until just combined.
Step 5: Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk. Begin and end with the dry ingredients (e.g., 1/3 dry, 1/2 buttermilk, 1/3 dry, 1/2 buttermilk, 1/3 dry). Mix until just combined; be careful not to overmix the batter.
Step 6: Gently fold in the diced fresh rhubarb until evenly distributed throughout the batter.
Step 7: Pour the batter into the prepared loaf pan and spread evenly.
Step 8: Bake for 50-60 minutes, or until a wooden skewer or toothpick inserted into the center comes out clean. If the top browns too quickly, loosely tent the loaf with aluminum foil.
Step 9: Remove the loaf from the oven and let it cool in the pan for 10-15 minutes before carefully transferring it to a wire rack to cool completely.
Step 10: While the loaf cools, prepare the glaze. In a small bowl, whisk together the powdered sugar and 2 tablespoons of fresh lemon juice until smooth. If the glaze is too thick, add more lemon juice, a teaspoon at a time, until it reaches a pourable consistency. If too thin, add a bit more powdered sugar.
Step 11: Once the loaf is completely cool, drizzle the lemon glaze generously over the top. Allow the glaze to set for about 10-15 minutes before slicing and serving.