Cake: Whisk the egg and cold water together, then whisk in the lemon juice.
Put the margarine in a large mixing bowl and sift in the flour. Using a knife or your fingers combine until the mixture resembles coarse breadcrumbs. Add the egg mixture and combine to form a smooth dough.
Roll the dough out into a log and cut into nine equal pieces. Roll each part into a ball, cover with plastic wrap and chill for one hour or overnight if possible.
When ready to bake, preheat the oven to 180 degrees C.
Dust a sheet of aluminium foil with flour. Lightly flour one of the dough balls, place on the foil and roll into a circle about 22cm in diameter. Prick all over with a fork and transfer the dough on the foil to a baking tray. Bake 10-15 minutes until golden brown. Repeat with the other dough balls.
While still warm, trim the pastry into smooth circles, using a plate or the bottom of a springform cake tin to guide you. Save the trimmings.
Filling: Beat the butter until light coloured and fluffy. Add the sweetened condensed milk and beat until smooth.
To Assemble: Place a pastry circle on a serving plate, spread with some of the filling then lightly sprinkle with crushed walnuts. Cover with another pastry circle and repeat, until all the pastry circles are used. Spread the top with the last of the filling, but don't add walnuts.
Crush the reserved pastry trimmings into crumbs and sprinkle over the top and sides of the torte. Chill overnight before serving.