Preheat oven to 180 degrees C. Grease and flour a 20x30cm pan.
Sift together the flour, baking powder and bicarb soda into a large bowl. Make a well in the centre and pour in the sweetened condensed milk, orange juice, vanilla and melted butter. Mix well and pour into prepared pan.
Bake in the preheated oven for 30 to 45 minutes, or until a toothpick inserted into the centre of the cake comes out clean. Allow to cool.