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To make the cookies, put the butter, sugars, treacle, egg and vanilla in a large bowl and mix together with an electric whisk until smooth and combined. In a separate bowl, mix together the remaining ingredients except the demerara. Add the dry ingredients to the egg mixture and mix until a very sticky dough is formed. Cover with cling film and chill for at least 4 hrs.
Heat oven to 180C/160C fan/gas 4 and line two baking trays with baking parchment. Roll the cookie dough into balls, about a tablespoon in size, then roll in the demerara sugar. Place on the lined trays, leaving about 2.5cm space between cookies.
Bake for about 14 mins or until just lightly browned around the edges, swapping the trays over halfway through cooking. Allow to cool on the trays for 10 mins before transferring to a wire rack to cool completely.
To make the filling, beat together the mascarpone and lemon curd in a bowl until smooth and creamy. Transfer to a piping bag fitted with a plain round piping tip. Pipe a layer of the cream onto the base of half the cookies and sandwich together with the other half. Will keep for 3-4 days in a sealed container in the fridge – the texture will turn soft and cakey (this is no bad thing).