Cake: Preheat oven to 180 degrees C. Grease a 23x12cm loaf tin.
Measure the flour, sugar, baking powder, bicarb soda, 1 1/2 teaspoons cinnamon, salt, buttermilk, oil, eggs and vanilla into large mixing bowl. Beat 3 minutes. Pour into prepared loaf tin. Smooth top.
Topping: Combine the topping ingredients; caster sugar, cinnamon and butter, mixing until crumbly. Sprinkle topping over smoothed batter. Using knife cut in a light swirling motion to give a marbled effect.
Bake for about 50 minutes. Test with a skewer; when inserted it should come out clean. Remove cake from loaf tin to a wire cooling rack.