Preheat oven to 180ºC. Grease and line a square brownie pan.
In a bowl, combine melted butter, Natvia, molasses and egg. Whisk together instant coffee and boiling water, and add this to the other wet ingredients.
Sift flours and cocoa powder into wet ingredient. Add one cup of chopped walnuts and stir mixture to combine.
Pour into prepared tin and bake for 30 minutes. Set aside to cool completely in the tin.
To make the topping, melt chocolate in the microwave, or over a bain-marie. Keep the slice in the tin, and pour over dark chocolate. Sprinkle with walnuts. Place into the fridge until chocolate topping has set. Cut into squares and serve.