Preheat the oven to 180℃/170℃ fan/gas mark 4. Lightly oil and line the base of a 23cm springform cake tin
Place the cocoa powder in a small bowl and slowly add the boiling water, mixing until smooth.
Beat the butter and sugar together until pale and fluffy. Then add the eggs one at a time beating well after each addition, then beat in the vanilla extract.
Beat in ½ teaspoon vanilla extract followed by the cooled cocoa powder mixture and mix well. Fold in 275g (10oz) flour.
Spoon the mixture into the prepared tin and level the top then make a slight dip in the centre. Bake for about 45 minutes until the cake springs back when lightly pressed.
Allow to cool in the tin for 5 minutes, then loosen the edge with blunt knife and turn out onto a wire rack to cool completely. Wash and dry the cake tin and line the edge with baking parchment.
When the cake is completely cold, use a sharp serrated knife to level the top of the cake if necessary and cut into two equal layers. Place one layer back into the tin.