Heat the butter, sugar, syrup, lemon juice and Grand Marnier gently together in a saucepan until the butter melts and the sugar has dissolved. Remove the pan from the heat, mix in the flour and leave to cool.
Preheat the oven to 180ºC/350°F Line 2 baking trays with non-stick baking paper with 6 circles of about 5 cm (2") marked on them, with good space between each. Alternatively, place this template under a Silpat – the circles will show through and you can slide the template out and reuse it. Place a teaspoon of the mixture in the centre of each circle and smooth out to the edges of the circle with a wet flat knife or back of a spoon. Spread them thinly, but not so thin that there are holes. Bake for 8-10 minutes, or until lightly browned. To ensure enough time to roll the brandy snaps, put one tray into the oven 5 minutes before the other.
Remove the brandy snaps from the oven and cool on the baking tray for a few seconds, then lift the cookies off with a palette knife and roll around wooden spoon handles. Don’t wrap them tightly around the handle – the snap will dangle off it, and the only bit of the snap that needs to be held tightly against the handle is the overlap: press that join firmly to seal. Then slide off the handle and place on a wire rack. If the snaps become too hard to roll, pop them back in the oven for a few seconds. Repeat with the remaining mixture. Snaps can also be rolled into cones.
Dice the strawberries and taste: add a little sugar if they are not sweet and juicy enough. Let them sit while you fill the snaps, so that they release a little juice.
To fill the brandy snaps, stir together the filling ingredients and spoon into a piping bag fitted with a small star nozzle and pipe into each end of the biscuits.
Top with a spoon or two of strawberries, and a sprig of mint if available. Serve as soon as they are filled. Leftover (ha!) unfilled cookies can be stored in air-tight containers for up to two weeks (if they last that long).