Before you begin, bring all of the cold ingredients to room temperature: set out the soured cream, butter and the eggs (separate the eggs while they are still cold) for about half an hour before making the cake.
Preheat an oven to 170 C / Gas 3. Grease and lightly flour a 25cm tube cake tin. Sift together the flour, bicarb, salt and cocoa. Combine the soured cream and vanilla extract in a separate bowl.
Beat the butter with an electric mixer in a large bowl until creamy, about 30 seconds. Gradually add 300g of sugar and beat on high speed until light and fluffy. The mixture should be noticeably paler in colour. Add the room-temperature egg yolks one at a time, allowing each yolk to blend into the butter mixture before adding the next.
Pour in one third of the flour mixture and mix on low speed until combined. Blend in half of the soured cream mixture. Continue adding the flour mixture alternately with the soured cream, mixing until just incorporated. Fold in the chocolate chips.
Beat the egg whites and cream of tartar in a clean mixing bowl with clean beaters until soft peaks form. Gradually add the remaining sugar and beat until medium-stiff peaks are formed (lift your beater or whisk straight up: the tip of the peak formed by the egg whites should curl over slightly). Use a whisk or rubber spatula to fold 1/3 of the egg white mixture into the cake mixture to lighten it.
Fold the remaining egg white mixture into the mixture until combined. Pour the cake mixture into the prepared tin and spread it evenly. Bake the cake for about 1 hour and 15 minutes, or until the cake springs back when you touch it lightly and a skewer inserted in the cake comes out clean.
Let the cake cool in the tin for ten minutes before inverting the cake onto a wire rack to cool completely.