Preheat the oven to 180 C. Line a 9x13 inch baking pan with parchment paper or grease it lightly with butter.
Melt butter and chocolate in microwave, in short burst of 30 seconds, until melted. Alternatively one can melt in a heat proof bowl over simmering water. Allow to cool.
Sift the cocoa powder, flour and baking powder in a separate bowl.
In a separate bowl, Mix eggs and sugar with an electric mixer until creamy.
Add in finely grated apple and mix.
Add in the melted chocolate and butter. Add in Vanilla essence and Keep mixing. (The chocolate butter mixture should be cooled completely)
Add the sifted cocoa powder, flour and baking powder and slowly mix everything just until combined.
Add in the walnuts and mix with a spatula. (One can top it up with walnuts too, I added in the batter)
Pour into the prepared pan and bake for 35 minutes. Keep an eye after 25-30 minutes.
Let it cool down completely, cut into squares and enjoy with a cup of milk or coffee.