Place the eggs, sugar, oil and milk in a large bowl, and mix until smooth using a handheld mixer (I use a K-Mix).
Fold in half the flour and mix until combined.
Pour half the mixture into a second bowl. Add the vanilla bean paste or extract, half of the remaining flour and half the baking powder and stir it in.
To the first bowl, add the cocoa powder, rest of the flour and the rest of the baking powder. Stir until combined.
Line your slow cooker either with baking paper or a large cake case (I use two cakes cases to add an extra layer of protection for the sides of the cake).
Using two dessert spoons, spoon in first a large spoonful of the vanilla cake mixture, followed by a large spoonful of the chocolate cake mixture directly on top of it. Keep repeating until you have used all the cake mixture from each bowl.
Turn your slow cooker onto high, place a clean tea towel under the lid (fold up the sides so it doesn't hang down) and put the lid on. Bake for approximately 1.5 hrs on high, or until a skewer or cake tester comes out clean from the deepest part of the cake. Watch out for burning, if you think it might be burning, rotate the bowl inside the slow cooker and/or turn to low for the rest of the time.