In another large mixing bowl, cream together the softened butter and the sugar. Add the egg and the vanilla extract and mix well.
Mix together the flour, bicarbonate of soda and salt. Gently fold in half of these dry ingredients, followed by half of the Greek yoghurt. Repeat with the remaining ingredients.
Fold in the cooled cooked rhubarb. Pour the mixture into the prepared cake tin. Sprinkle with the streusel mixture. Place in the oven and bake for 40-50 minutes or until golden, firm and a skewer inserted comes out clean. Leave to cool completely before cutting into 16 squares.