Mix the softened butter with the sugar. Once it gets creamy, add the flour, and knead until it forms a ball.
Put some wax paper in the mould, and put the dough in it. Spread it out with your hands. Bake 25 minutes at 350°F (180°C).
Melt the butter and add the sugar. When this mix starts to boil, add the condensed milk and stir.
Then, at medium heat, constantly stir the mix until it gets light brown, it should take around 10 minutes.
Pour the toffee directly on the shortbread in the mould, then put in the fridge for 1 hour.
Melt the chocolate, and pour it on the toffee. Put in the fridge until it gets hard again.
Finally, take it all out of the mould, and cut it in squares or in rectangles.
There you are, your homemade Twix are ready!