Sift over half of the flour-cocoa mixture, and gently fold in with a large spatula. Repeat to fold in the rest, then gently fold in the raspberry purée. Scrape into a 20-22cm round and about 8-10cm deep ring or savarin tin. Tap the tin once, sharply, on the kitchen surface to remove any air bubbles, then smooth the top and bake for 45 minutes. Invert the tin onto a cooling rack and cool for 1 hour before using a palette knife to help you remove the cake from the tin to allow it to cool completely – you’ll need to do a bit of gentle prising out but it will come away eventually and any imperfections will be hidden by the frosting.