2) Preheat the oven to 180C/Gas 4. Generously brush a 23cm round by 5cm deep ring tin with butter. In a medium bowl whisk the flour, bicarbonate of soda, salt, nutmeg and mace together and set it aside.
3) In a stand mixer fitted with the paddle attachment, or with an electric hand-held mixer, cream the butter and sugar on medium speed until light and fluffy. Add the eggs, one at a time, mixing until incorporated.
4) Mix the vanilla with the sour cream. Add the flour mixture in 3 parts to the butter mixture, alternating with the sour cream in 2 parts until just combined.
5) Spread half the batter into the prepared tin and sprinkle with half the streusel. Spoon the remaining batter on top and spread it out with a spatula. Scatter the remaining streusel over the top of the batter.
6) Bake the coffee cake for 1 hour 10 minutes until golden brown, or until a cocktail stick inserted into the centre of the cake comes out clean. Cool on a rack for 10 to 15 minutes before serving.