Line your slow cooker with baking paper or line a cake tin and place in slow cooker.
In a large bowl using an electric mixer, whisk egg yolks and sugars together until pale in colour and creamy then add in cooled melted butter until combined.
Sift in flour and whisk on low slowly adding milk until combined and lump free. Add in caramel and vanilla and mix to combine.
In a separate bowl whisk egg whites to soft peaks.
Combine egg yolk mix and egg whites.
Place in to slow cooker and cook on high for 2 hours with a tea towel under the lid. (Check after 1.5 hours depending on your cooker)