1Put about an inch of water in the bottom of your slow cooker and turn on High.
3Butter and flour the sides of a 20cm (8in) diameter spring-form/loose-bottomed cake tin and line the base with greaseproof paper.
5Cream the butter in a large bowl or in an electric food mixer until soft. Add the sugar and beat until the mixture is light and fluffy.
7Whisk the eggs and vanilla extract together in a small bowl. Gradually add the eggs to the butter mixture, beating all the time. Sift in the flour and baking powder and fold in gently to mix, then add the milk and mix gently to combine.
9Tip half of the cake mixture into another large bowl and, into this bowl, fold in the sifted cocoa powder.
11Place the cake mixtures into the prepared tin by alternating spoonfuls of the vanilla batter with the chocolate batter, then with a skewer or similar implement, gently draw swirls through the cake mixture to marblise it. Dont over mix or you wont have that wonderful marble effect.
13I then place the tin on a cooking rack in the Slow Cooker and place a tea towel under the lid. You may also elevate the tin by placing it on upside down egg rings, or balls of foil.
15Cook for 2 hours on High or until a skewer inserted into the middle comes out clean.
17Turn the cake out onto a wire rack and allow to cool.
19Slow cookers will vary in time, so check after 1:30hrs
Submitted by Roslyn Potter (Recipe inspired by Rachel Allen Bake)