Preheat the oven to 375 degrees F. Line a baking dish with wax paper and lightly grease the paper. Set aside.
Place the butter /margarine and 1 cup chocolate chips in a bowl. Melt together in a microwave for approximately 45 sec. Stir the mixture until smooth.
Add the sugar, mixing well to incorporate. Beat in the eggs one by one, ensuring the eggs are well incorporated into the chocolate mixture.
Mix in the cocoa powder and vanilla.
Sift together the flour, baking powder and salt. Add to the chocolate mixture and gently stir in. Mix in the remaining chocolate chip. Pour into the prepared baking tin, evenly spreading mixture.
Bake in a preheated oven for 40 – 45 minutes or until a toothpick inserted in the centre comes out clean. Remove from the pan and place on a wire rack to cool, peeling off the wax paper. Cut into squares and enjoy.