In a large bowl, sift together the flour, cocoa, baking powder, salt, and bicarbonate of soda. Add the granulated sugar and whisk until well combined.
In a separate medium bowl, whisk together the eggs, milk, sour cream, vegetable oil, tahini, and vanilla. Add the wet ingredients to the dry ones and whisk until blended and no streaks of flour remain. Whisk in the boiling water, stirring until smooth.

Scrape the batter into the tin, smooth the top, and bake for 20 minutes. Scatter the crumble on top of the cake and continue to bake until a cake tester inserted in the centre comes out clean , 20 - 25 minutes. Cool for at least 30 minutes, then remove from the tin.

If making the melted chocolate, 5 - 10 minutes before serving place the chopped chocolate in a microwave-safe bowl and microwave on high in 30-second intervals, stirring in between, until the chocolate is fully melted. Drizzle the chocolate over the top and sprinkle with sesame seeds before serving, if desired.

Recipes extracted from ‘Eitan eats the World’ (Clarkson Potter)