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To make the base mixture, tip the flour and icing sugar into a bowl with a pinch of salt and stir together. Add the egg yolks and butter to the bowl and mix everything together slowly to make a dough. Knead briefly until smooth on a flour-dusted worksurface. Form the dough into a roll about 2½-3cm across. Wrap in foil and chill for 1 hour.
Heat the oven to 160C/fan 140C/gas 3. Cover 2-3 baking sheets with baking parchment. Cut the dough roll into 45 thin slices and put the slices on the sheets. Put one toffee or caramel (upside down if using cups) on each.
For the meringue, put the 3 egg whites in a bowl and begin to beat to soft peaks. Beat in the sugar and a pinch of salt in batches and beat for 4-5 minutes until you have stiff glossy meringue peaks. Spoon into a pastry bag with plain nozzle and pipe the meringue in a spiral, starting at the base, all the way up the toffee centre until it is completely covered. Smooth over any gaps. Repeat with the remaining cookies. Bake for 18 minutes. Cool completely.