Preheat the oven to 350°F / 176.667°C, Using a springform cake tin, spray with any non-stick cooking spray.
Using a handheld or stand mixer fitted with a paddle or whisk attachment beat the butter and sugar together on high speed until smooth and creamy, about 3 minutes.
Scrap down the sides and up the bottom of the bowl with a rubber spatula as needed, then sift the flour and baking powder together.
With the mixer on low speed, add the dry ingredients just until combined, with the mixer still running on low, pour in the milk and melted butter and mix just until combined.
You may need to whisk it all by hand to make sure there are no lumps at the bottom of the bowl, the batter will be slightly runny, pour batter evenly into the springform pan.
Place in preheated oven for 20 to 30 minutes.
Making the Syrup
While the cake is in the oven, place the orange juice in a saucepan on medium heat, bring to the boil, add the butter and the cornflour that has been mixed with a little water to form a paste.
Slowly stir this into the orange juice stirring until the sauce is slightly thick, set aside to cool.
When the cake is ready, remove it from the springform pan, place it on the serving plate, using the back of a large spoon, slowly pour the sauce over the cake while it is warm.