Preheat an oven to 180 degrees C. Grease and flour a 23x33cm cake tin. Combine 2 1/2 cups of flour, baking powder and bicarb soda in a bowl.
Beat the 190g butter and 1 1/4 cups of sugar with an electric mixer in a large bowl until light and fluffy. The mixture should be noticeably lighter in colour. Add the room-temperature eggs one at a time, allowing each egg to blend into the butter mixture before adding the next. Beat in the vanilla with the last egg. Pour in the flour mixture alternately with the sour cream, mixing until just incorporated. Batter will be thick. Pour the batter into prepared tin.
Mix 60g of butter, 1/2 cup of flour, 1/4 cup of sugar and cinnamon in a small bowl until it resembles a coarse crumble. Sprinkle over the cake batter.
Bake in the preheated oven until a toothpick inserted into the centre comes out clean, 30 to 35 minutes. Cool in the tin for 10 minutes before removing to cool completely on a wire rack.