marbled caramel chocolate slice

I made these for the first time this year at my holiday cookiepalooza. I was able to get some out of the house (with strenuous objections from Leo) and take them to work. They were everyone's favorite cookie in the assortment I brought in.

For the base:
2 1/4 cups plain flour
a scant half cup of caster (superfine) sugar
3/4 cup of unsalted butter, softened.

For the filling:
7 TBSP of unsalted butter, diced
a scant half cup of light muscovado (brown) sugar
2 14oz cans of sweetened condensed milk

For the topping:
3 1/2 oz plain, semisweet chocolate
3 1/2 oz milk chocolate
2 oz white chocolate

1. Preheat oven to 350F. Line and lightly grease a 13x9 jelly roll pan. Put the flour and caster sugar in a bowl and rub in the butter until the mixture resembles fine breadcrumbs. Work with your hands until the mixture forms a dough.
2. Put the dough into the prepared pan and press it out with your hand to cover the base. Then, use the back of a tablespoon to smooth it evenly into the pan. Prick all over with a fork and bake for about 20 minutes, or until firm to the touch and very light brown. Set aside and leave in the pan to cool.
3. To make the filling, put the butter, muscovado sugar, and condensed milk into a pan and heat gently, stirring until the sugar has dissolved. Reduce the heat and simmer the mixture very gently, stirring constantly for about 5-10 minutes until it has thickened and turned slightly darker. Take care that the mixture does not burn on the base of the pan, as this will spoil the flavor. Remove from the heat.
4. Pour the filling mixture over the cookie base, spread evenly and then leave until cold.
5. To make the topping, melt each type of chocolate separately in a heatproof bowl set over a pan of hot water. Spoon lines of plain and milk chocolate over the set caramel filling.
6. Add small spoonfuls of white chocolate. Use a skewer to form a marbled effect on the top.
7. Let set, then heat a sharp knife in boiling water and cut into bars.

Makes about 24