Preheat an oven to 180°c/160°c fan (356°f/320°f).
Melt the butter (either in a microwave or in a pan on the stove) then transfer to a bowl & leave to cool slightly.
Brush a 12 hole muffin tray with melted butter then set aside.
Next, take a sheet of the filo pastry & cut it into 8 evenly sized squares.The squares of pastry should be roughly 11 cm wide. If you are using smaller sheets of filo, cut them into 4 squares. If they're larger sheets, cut them into 6 or 8.I like to use a pizza cutter to cut the filo but a sharp knife will work as well! Take 4 of the filo squares & set the others aside.Make sure to keep any unused filo pastry covered with a damp tea towel, to keep it from drying out. Lay a filo square onto your worksurface then brush the top with melted butter. Take another pastry square & lay on top, at a slight angle then brush with butter.See the post above for a visual guide... Lay another square on top, at an angle then brush with butter. Lay the final square of filo on top, at an angle but don't brush with butter.
Press the stack of filo into one of the muffin tray holes then fill two teaspoons of mincemeat (the mincemeat should be just under the top of the tin).
Add a light sprinkle of the cold streusel to the top of the mincemeat then fold over the overhanging filo pastry so that it's level with the muffin tray hole.
Brush the top of the filo with melted butter then repeat the assembly process with the remaining pastry & mincemeat.
Bake in the preheated oven for 25-30 minutes until the pastry is crisp & the streusel is golden brown. If the pastry looks like its' getting too brown, cover the tray loosely with foil.
Once cooked, let the mince pies cool in the tin for a couple of minutes then use a small palette knife to transfer them to a cooling rack. Leave to cool completely then dust with icing sugar & serve.