Add butter and chocolate in a microwave safe bowl. Heat in the microwave for 20 - 30 seconds. Stir the melted butter and chocolate with a spatula to make sure the mixture is silky smooth and lump free.
Add yogurt, sugar, and vanilla essence in the liquid chocolate mixture. Whisk for 2 - 3 minutes to combine all the ingredients nicely. Once combined, set the bowl aside.
Sift the flour, baking powder, cocoa powder, and coffee powder together in a separate bowl. Mix using a spatula.
Add the sifted flour mix into the liquid chocolate in small batches. Gently cut and fold the batter using a spatula untill all moistened and combined well. Do not mix the batter for too long.
Add chopped walnuts and fold them into the batter.
Preheat the oven to 180 degree celcius. Grease an 8 inch square tin and line it with a parchment paper.
Pour the brownie batter into the prepared tin and bake for 20 - 25 minutes. Check the brownies for doness via inserting a toothpick after 20 minutes.
Remove from the oven and place on a wire rack to cool. Cut into squares and serve hot with vanilla ice cream.