Preheat the oven to 350°F/180°C. Line a cookie sheet with baking paper or a silpat.
Thaw the frozen blackberries, either in a pan on the stove or in the microwave, then puree with a hand held immersion blender. Set aside to cool. You should have ½ cup of blackberry puree.
In a medium bowl, cream the butter and sugar with an electric mixer until light and fluffy. Add the cooled blackberry puree and vanilla and beat well.
In a small bowl, whisk together the flour, baking soda, and salt, then add to the blackberry mixture and stir by hand to combine. Fold in the chocolate chips (reserving ¼ of them to add to the cookies after baking.
The dough will be very soft, so cover the dough and chill in the fridge for at least 30 minutes.
Roll tablespoonfuls of the dough into balls, place them onto the cookie sheet and press down slightly.
Bake the cookies for 8-10 minutes or until the edges are set. Cool on the tray for a few minutes, then press the remaining chocolate chips onto the cookies. Transfer to a cooling rack to cool completely.
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Notes
Although the batter is purple, it turns blue-grey when it’s chilling in the fridge. When the blackberries react with the baking soda they turn blue naturally. The color is consistent inside and out and darkens slightly the next day.If the cookies are overcooked, the edges might lose a little of the blue color.Make sure you chill the dough for at least 30 minutes before baking. This will help keep them from spreading. Reserve ¼ of the chocolate chips to place on top for baking for a beautiful blue cookie that’s perfectly studded with chocolate chips. Don’t overwork the cookie dough or it may change the texture to be a bit tough and chewy.