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1
Preheat the oven to 350 F/180 C and grease and line an 8x8 square baking tin.
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2
Place the cookies into a mini food chopper and blitz until you have 2 1/2 cups of crumbs. Tip the cookie crumbs into the greased tin and smooth until level using a spatula or wooden spoon. Place it in the oven for 8-10 minutes until ever so slightly browned on the edges and puffy. Remove from oven. Leave it to cool whilst you make the cake layer.
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3
Sift the flour, cocoa powder and baking powder together in a small bowl, leave to one side.
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4
Place the butter and sugar into a large bowl or the bowl of your stand mixer and beat until light and fluffy, about 2-3 minutes on medium-high speed. Mix in the eggs and vanilla, scraping the sides when necessary. Mix until well incorporated, 1-2 minutes on medium-high speed.
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5
Add in the flour mix and mix on low until just incorporated, then add in the milk and mix until well incorporated and smooth.
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6
Pour the batter on top of the cooled cookie base and smooth with a spatula or wooden spoon.
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7
Place in the oven for 25-30 minutes, then turn the oven down to 325 F/160 C and bake for a further 10-15 minutes. In the final 10-15 minutes keep checking to make sure the cake doesn't burn. If the edges are getting a bit brown, cover them with foil. Cake is done when a skewer inserted in the centre comes out clean. Remove from oven.
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8
Let it cool in the tin completely, before removing and slicing into 9-12 bars. Cake bars will keep in an airtight container, at room temperature, for 3 days
