Preheat the oven to 160°C. Line a 20cm square cake tin with foil, leaving a 2.5cm overhang.
Sift the flour and 1 cup (90g) cocoa powder into a large bowl. Heat the buttermilk and espresso in a small saucepan over a low heat and stir until the coffee has dissolved.
Mix the caster and brown sugars, apple sauce and vanilla essence in a bowl. Pour in the hot buttermilk-coffee mixture, then add the flour mixture and stir until just blended.
Whisk the egg whites until soft peaks form. Fold the egg whites and chocolate chips into the flour mixture.
Turn the batter into the tin. Bake for about 45 minutes or just until set (do not overbake). Cool in the tin on a wire rack for 15 minutes, then lift out onto the rack to cool completely. Sift the icing sugar and remaining cocoa powder over the cake. Cut into 4cm squares to serve.
Some more ideas…
*Add 1 tbsp coffee liqueur to the mixture for an adult's only version. *Substitute coarsely grated dark chocolate for the chocolate chips.
By using apple sauce, this recipe needs no oil. The slight apple flavour the sauce imparts complements the coffee-chocolate taste combination.