Preheat the oven to 160°C/325°F/gas 3. Arrange the pecans in the base of the prepared tin, pressing them onto the greased paper.
Sift the flour with the baking powder and bicarbonate of soda; set aside.
Put the chocolate into the bowl of a food-processor with the cocoa and 100g of the sugar. Process until the mixture looks sandy-textured. With the machine running pour in the hot coffee through the feed tube and process until the chocolate has melted. Stop the machine and scrape down the sides of the bowl.
Add the eggs and the rest of the sugar, and process for 30 seconds.
Scrape down the bowl again, then add the butter and run the machine for a minute. Scrape down the bowl once more. Add the soured cream and the flour mixture, and process just until the mixture becomes a smooth, streak-free batter.
Spoon the mixture into the tin on top of the nuts. Bake for about 45 minutes or until the sponge has started to shrink away from the sides of the tin, and a skewer inserted into the centre comes out clean. The top will have a crust but will be only just firm.
Remove from the oven. Run a round-bladed knife around the inside of the tin to loosen the sponge, then leave to cool completely in the tin. When cold, set a board on top of the tin and invert. Lift off the tin and the lining paper – the nutty base of the cake is now the top.
To make the icing put the chocolate into a small, heavy-based pan with the butter, icing sugar and coffee. Set on the lowest possible heat and stir gently until melted and smooth. Remove from the heat and leave until thick enough to spread. Cover the top of the cake with the icing and leave to set before cutting. Store in an airtight container and eat within 4 days.
This recipe is brought to you from 'The Great British Bake Off' by Linda Collister published by Random House