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 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 and fluffy, about 3–4 minutes.
04 - Add the eggs one at a time, beating well 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. Do not overmix.
06 - Gently fold the diced rhubarb pieces into the batter until evenly dispersed.
07 - Spoon the batter into the prepared pan and smooth the top with a spatula. Bake for 45–55 minutes, or until a wooden skewer inserted into the center comes out clean.
08 - While the cake bakes, whisk together the 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 soak in.
10 - Let the cake cool in the pan for 15 minutes, then turn out onto a wire rack to cool completely. Dust with powdered sugar and sprinkle with extra lemon zest before serving, if desired.