1. You need a 9" x 13" pan. Heat the oven to 180c.
2. Beat eggs in a bowl with cocoa, salt, baking powder, espresso powder and vanilla extract for 4 minutes.
3. Melt butter over low heat and add sugar, stirring to combine. It should become shinny as you stir it.
4. Add this butter to the egg/ cocoa mixture, stirring until smooth.
5. Add flour and chocolate chips.
6. Spoon butter into the lightly greased baking pan and bake for 30 minutes. A tester poked in should come out clean.
7. Cool before cutting and serving.