Preheat the oven to 175°C. Coat a 2-quart baking dish with nonstick cooking spray.
Whisk together both flours, 1/2 cup of the brown sugar, 1/4 cup cocoa, baking powder, and salt in a large bowl. In a separate bowl whisk 1/2 cup of the malt powder with the milk, egg white, butter, and vanilla until the malt dissolves. Stir into the dry ingredients until only a few small lumps remain and spread the batter in the prepared baking dish.
Combine the remaining 1/4 cup of brown sugar with the remaining 2 tablespoons of cocoa and sprinkle over the batter. Stir the remaining 1/2 cup malt powder into the boiling water and pour it over the batter. Bake until the cake springs back when lightly touched and a toothpick inserted in the center comes out with a few moist crumbs, 30 to 35 minutes. Cool for 30 minutes on a wire rack. If desired, dust the top of the cake with icing sugar and immediately spoon the warm cake and sauce onto plates or shallow bowls.
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Nutritional analysis per serving
Calories 203; Total Fat 6g (Sat Fat 2.5g, Mono Fat 1g, Poly Fat 0g) ; Protein 4g; Carb 37g; Fiber 1g; Cholesterol 12mg; Sodium 294mg