1. Preheat oven to 350F, grease and line a 9 springform pan.
2. Combine cocoa and water, whisking smooth. Set aside.
3. In a bowl, whisk together flour, baking soda, baking powder and salt. Set aside.
4. Cream butter and sugar until fluffy.
5. Add eggs and vanilla, beating well.
6. Beat in cocoa mixture until completely blended.
7. Starting and ending with the flour, alternate additions with sour cream.
8. Bake 28-30 minutes, until tests done. Do not overbake.
9. Cool completely in tin before unmoulding.