Directions

1. Heat oven to 180C/160C fan/gas 4. Grease a 20 x 30cm cake tin and line with baking parchment. Mix the flours, cocoa powder, baking powder, bicarbonate of soda and sugar in a large bowl with 1/2 tsp salt. Shake the bowl a few times to encourage any large lumps of sugar to come to the surface and crush these with your fingertips. Mix the butter, buttermilk, eggs and vanilla in a jug and pour into the dry ingredients, add the courgette and chopped chocolate too. Stir everything together, then scrape into the tin. Bake for 45-50 mins until the cake is risen, and a skewer inserted to the centre comes out with just a few moist crumbs, but no wet cake mixture. Leave to cool in the tin for 15 mins, then transfer to a wire rack to cool.
2. To make the icing, put the chocolate in a heatproof bowl, suspended over a pan of barely simmering water. Stir every now and then until melted. (Alternatively, melt in the microwave, stirring the chocolate every 20 secs so it doesn't burn.) Remove the bowl from the heat and leave to cool a little. Meanwhile, put the butter and icing sugar in another large bowl and mash with a spatula to combine (this will prevent you covering the work surface in icing sugar), then whisk with a hand mixer until smooth. Sift in the cocoa and pour in the melted chocolate and milk, then mix again until smooth.
3. If using the rye bread, heat oven to 160C/140C fan/gas 3. Blitz the bread in a food processer to chunky crumbs, then tip onto a baking tray, spreading them in an even layer. Bake for 15 mins, stirring halfway, until lightly toasted and crispy. Spread the chocolate icing over the cake, then scatter with rye crumbs and sea salt. Cut into squares to serve.