For the filling: place the spread in a bowl with the cornstarch and mix until it forms a smooth creamy texture. Transfer to a piping bag with a 1cm diameter piping tip.
Line a tray with baking paper and pipe the filling into ~5cm strips. Transfer to the freezer and freeze for at least an hour.
For the dough: place all the dough ingredients except for the flour in the bowl of a stand mixer with the beater blade and beat until it forms a light and creamy mixture. Add the flour and mix until it forms a ball of dough and no more.
Wrap the dough tightly with cling film and chill in the fridge for at least 45 minutes (up to two days).
Lightly flour the work surface and divide the dough into three. Roll each to a thickness of about 3mm and cut into 6cmx6cm squares.
Remove the frozen filling nuggets from the freezer and place one filling nugget in the centre of each square. Roll and seal and tighten the edges. If the filling nuggets soften too much, return them to the freezer.
Place the biscuits into a suitable container (using parchment paper to separate the layers) and freeze for 20-30 minutes (up to two months).
Preheat the oven to 180°C and line two trays with parchment paper. Place the biscuits on the sheet pans, spaced 1-2cm apart.
Bake for 17-18 minutes, until golden. Remove and let cool, then dust with icing sugar.