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1
Preheat oven to 325 F and grease and line an 8x8 inch square pan.
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2
Make the shortbread first: Place the butter, flour, cocoa powder, cornflour, icing sugar and vanilla into a large bowl, or the bowl of your stand mixer and mix until you have moist crumbs. This will take about 2 minutes on medium speed.
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3
Tip the shortbread crumbs into your prepared pan and press into the tin until flat and level. Place tin in the oven for 15-20 minutes until slightly crisp on top-it will also look slightly lighter than before baking.
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4
Whilst the shortbread layer is baking, place the condensed milk, egg yolks, pureed blackberries, salt and flour into a large bowl and beat on medium speed until light and smooth, about 1 minute.
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5
When the shortbread is done, remove tin from the oven. Pour the blackberry mixture on top and place it back in the oven for 25-30 minutes until lightly golden around the edges and an inserted skewer comes out with just a tiny amount of mixture on it. When done, the middle will still wobble ever so slightly.
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6
Leave to cool completely in the tin, then slice into 12-16 bars. Garnish with a fresh blackberry and a dusting of icing sugar, if desired.
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7
Bars will keep in an airtight container, at room temperature for 2 days.
