Cooking oil spray, for greasing
2 1/4 cups plain flour
1/4 cup cocoa powder
1/3 cup brown sugar
275g unsalted butter, chopped, softened
2 Tbsp water
3/4 cup caster sugar
2 tsp vanilla extract
2 tsp baking powder
100g dark chocolate, chopped
Double cream, to serve
Preheat oven to 180°C. Grease a 20cm square cake tin with cooking oil spray and line base and sides with baking paper.
To make chocolate crumble, sift 3/4 cup of the flour and 1/2 of the cocoa into a large bowl. Stir in brown sugar and 100g of the butter. Rub with fingertips until a crumbly mixture forms. Add water and mix gently, then set aside.
Put caster sugar and remaining butter in the bowl of an electric mixer and beat on medium, using paddle attachment, for 5 minutes or until light. Beat in eggs, 1 at a time, beating well after each addition. Stir in vanilla. Sift remaining flour and baking powder over top and mix gently until smooth.
Dollop 3/4 of the batter into prepared tin. Sift remaining cocoa into remaining batter and stir to combine, then fold in dark chocolate. Dollop amongst vanilla batter and smooth surface. Top with chocolate crumble, then bake for 40 minutes or until cooked when tested with a skewer. Cool in tin for 10 minutes, then transfer to a wire rack to cool completely. Slice and serve with double cream.