To make the cake, place the chocolate in a heatproof bowl. Set the bowl over a saucepan of simmering water, making sure the base of the bowl doesn’t touch the water. Melt the chocolate, stirring every so often.Set aside to cool slightly. Using a stand mixer fitted with the paddle attachment or electric whisk, beat the butter, sugar and vanilla until light and fluffy. Gradually beat in the egg yolks before stirring in the cooled, melted chocolate and malt extract. Fold in the flour, baking powder and malted milk powder to get a smooth, thick batter. In a medium bowl, whisk the egg whites and salt for 2 to 3 minutes, or until stiff. Take one-quarter of the egg whites and stir it into the chocolate batter to loosen it up. Carefully fold in the rest of the egg whites until the batter is smooth with no streaks. Scoop the batter into the cake pan and bake for 45 to 55 minutes, or until the cake is firm to the touch and a toothpick inserted in the centre comes out clean. If the cake is taken out too early, it may sink in the middle.