01 - Preheat oven to 350°F. Grease an 8-inch round cake pan and line the bottom with parchment paper.
02 - In a medium bowl, sift together the all-purpose flour, baking powder, and salt. Set aside.
03 - In a large mixing bowl, beat the softened butter and granulated sugar together using an electric mixer until pale, light, and fluffy.
04 - Add the eggs one at a time, mixing thoroughly after each addition. Stir in the lemon zest and vanilla extract until evenly distributed.
05 - Fold in half of the sifted flour mixture followed by half of the milk. Repeat with the remaining flour and milk, folding gently until just combined. Be careful not to overmix.
06 - Gently fold the diced rhubarb pieces into the batter, distributing them evenly throughout.
07 - Spoon the batter into the prepared cake pan and smooth the top with a spatula. Bake for 45 to 55 minutes, or until a skewer inserted into the center comes out clean.
08 - While the cake bakes, whisk together the fresh lemon juice and granulated sugar in a small bowl until the sugar has fully dissolved. Set aside.
09 - Remove the baked cake from the oven and immediately prick the surface all over with a skewer. Pour the lemon drizzle evenly across the hot cake, allowing it to seep into the holes.
10 - Let the cake cool in the pan for 15 minutes, then transfer it to a wire rack to cool completely. If desired, sprinkle with extra lemon zest or dust with confectioners' sugar before serving.