01 - 
                Set your oven to 175°C (350°F). Grease and flour a 9-inch round cake pan to prevent sticking.
              
              
              
                02 - 
                In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
              
              
              
                03 - 
                In a large bowl, beat sugar, eggs, vegetable oil, and vanilla extract until smooth. Stir in the milk.
              
              
              
                04 - 
                Gradually fold the dry ingredients into the wet mixture, stirring until just combined. Avoid overmixing.
              
              
              
                05 - 
                Gently fold in the chopped rhubarb until evenly distributed.
              
              
              
                06 - 
                In a small saucepan, whisk together milk, custard powder, and sugar over medium heat. Cook, stirring constantly, until the mixture thickens into a smooth custard.
              
              
              
                07 - 
                Pour the cake batter into the prepared pan and smooth the top. Pour the custard over the batter and swirl gently with a spoon.
              
              
              
                08 - 
                Place in the oven and bake for 35–40 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
              
              
              
                09 - 
                Allow the cake to cool for at least 15 minutes before slicing. Serve warm or at room temperature with whipped cream or ice cream.