Pre-heat the oven to 180°C (350ºF) and grease a 6 inch cake tin (I used a springform).
Melt the butter and set to one side.
Prepare the raspberry sauce by heating the berries and water together until the fruit softens. Mush them a little with a fork until they become a thick sauce. Set to one side.
Mix the flour, baking powder, baking soda and sugar together in a bowl.
Beat the egg then whisk in the Greek yoghurt, oil and vanilla extract along with the cooled butter. Pour into the dry ingredients and mix until just combined.
Pour into the prepared tin (the batter will be very thick) and then using a knife, swirl the sauce into the batter.
Bake for around 20 minutes, until the top is firm to the touch and an inserted skewer comes out clean.
Cool in the pan for ten minutes or so, then either turn out onto a wire rack, or remove the outside layer (if using a springform pan). Allow to cool completely and then slice and enjoy!