Rhubarb Lemon Drizzle Cake

Name: Rhubarb Lemon Drizzle Cake
Ingredients: 24 Tbsp. salted butter, softened, plus more for greasing; 1½ cups plus 1 Tbsp. superfine sugar; 3 Tbsp. finely grated lemon zest; 1 tsp. vanilla paste; 6 large eggs; 2¼ cups plus 3 Tbsp. all-purpose flour; ¼ cup plus 1 Tbsp. cornstarch; 1 Tbsp. baking powder; Kosher salt; ¼ cup plus 2 tsp. whole milk; 2 cups coarsely chopped rhubarb; ¼ cup plus 1 Tbsp. superfine sugar; 2 cups confectioners sugar; ¼ cup plus 2 Tbsp. fresh lemon juice; 3 cups plus 2 Tbsp. confectioners sugar; 18 Tbsp. salted butter, softened; 2 Tbsp. finely grated lemon zest; 2 Tbsp. fresh lemon juice; Lemon slices and edible flowers, for garnish