1) Preheat the oven to 180C/Gas 4. Butter and flour 23cm by 33cm baking pan.
2) In a small sauce pan melt the butter. Put the light brown sugar in a large bowl, add the butter, and stir to combine. Cool to room temperature. Beat in the eggs and vanilla.
3) In a medium bowl, whisk together the flour, baking powder, and salt. Add the flour mixture into the wet ingredients, mixing just until a smooth batter is formed. If using the chips, stir them into the batter.
4) Transfer the batter to the prepared pan. Bake until lightly browned and toothpick inserted in the center come out clean, about 20 minutes.
5) Cool the blondies in the pan slightly before inverting them onto a cooling rack. Cool completely. Cut into squares and serve.