Preheat the oven to 180C.
Grease and line a cake tin.
Mix all the cake ingredients in a bowl.
Pour the mixture into the cake tin and bake for about 50 minutes.
Check if the cake is done by inserting a skewer in the middle.
Let the cake cool in the tin before turning it out onto a wire rack to cool completely.
Mix mascarpone cheese with vanilla extract and icing sugar for the topping.
Spread the topping over the cooled cake and sprinkle chopped walnuts on top.
Store the cake in the fridge.