Sweet Potato Cake

Ingredients:

125g butter, softened
3 eggs, separated
150g caster sugar
1 tsp vanilla extract
1/2 cup cold mashed sweet potato
190g self-raising flour, sifted
1/3 cup yoghurt
zest of 1 lemon
3 tbsp cocoa powder

• Grease and line a 14cm x 21cm loaf pan.
• Preheat oven to 160 degrees C.
• Cream butter and sugar until light and fluffy.  Beat in egg yolks, lemon zest and vanilla extract until combined.
• Beat egg whites until firm peaks form.
• Stir in mashed sweet potato with 1/3 of the sifted flour and half of the yoghurt.  Then stir in remaining flour and yoghurt.  
• Fold in beaten egg whites.  
• Spoon 1/3 of the mixture into a separate bowl and stir in cocoa powder.  Combine well.
• Place the cake mixtures into the prepared pan by alternating layers of the vanilla batter with the chocolate batter.
• Bake for about 40 minutes or until cake is done.