Preheat oven to 180°C. Lightly grease and line a 22cm x 32cm lamington pan.
Place butter and chocolate in a saucepan and stir over low heat until the chocolate melts. Spoon the melted mixture into a mixing bowl and add sugar and essence. Mix in eggs. Add sifted flour, yoghurtand nuts. Beat with an electric beater until well combined.
Turn mixture out into the prepared pan and bake for about 30-35 minutes or until firm to the touch in the centre.
Set aside to cool completely before slicing into serving pieces.
Tip: As a guide, to test if the brownie is cooked through, a skewer inserted into the centre of the brownies will come out with slightly moist crumbs attached and also a crust will form on the top. This is an indication that the brownie is done correctly.