Preheat your oven to 180C / 350F / 160C fan. Line an 8x8 inch baking tin with baking paper.
FOR THE BASE
Set aside 6 cookies, then place the rest into a food processor and blitz to crumbs. Mix together with the 1/4 cup of melted butter then press the mixture into the base of your lined tin.
Break the remaining cookies into quarters and set aside.
FOR THE BLONDIES
Combine the flour, baking powder, salt and both sugars in a bowl and mix well until there are no lumps left. '
Add the melted butter, melted chocolate, eggs and vanilla and mix to just combine.
Spread the batter over the base as evenly and level as possible. Scatter the cut cookies over the top and press them in gently.
Bake for 35-40 minutes until the top is dry and golden and a toothpick comes out clean.
Leave to cool completely in the tin to room temperature before using the edges of the baking paper to lift it out. Then cut into slices and serve.
I use a standard Australian 20ml tablespoon (4 teaspoons worldwide)
For best results you should always weigh ingredients like flour and sugar. Kitchen scales like these are relatively cheap but if you can’t weigh the ingredients, use the spoon and level method (don’t scoop).
All ovens vary. What takes 40 minutes in my oven may take 35 or 45 in yours. Many ovens also don't heat evenly or may even run hot or cold so it's a good idea to keep an oven thermometer in the centre of your oven at all times. They're very cheap and are a good back up.