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Beat the margarine, sugar and vanilla using an electric whisk for 1 min until pale and smooth, then mix in the peanut butter and flour until fully incorporated. Bring together into a smooth dough with your hands.
Roll out the dough between two sheets of baking parchment until it’s about 1cm thick. If the dough is very soft (this will depend on the margarine used), chill for 45 mins until firmed slightly. Heat the oven to 160C/140C fan/gas 3 and line one large or two medium baking trays with baking parchment.
Cut circles out of the dough using an 8cm cookie cutter, placing the rounds on the lined baking trays. Press a few peanut halves, cut side-up, firmly into the tops of the biscuits. Bake for 12-15 mins until turning lightly golden around the edges. Leave to cool on the trays for 5 mins, then lift onto a wire rack to cool completely.