With the rack in the middle position, preheat the oven to 350°F (180°C). Butter an 8-inch (20 cm) square baking dish and line the bottom with a sheet of parchment paper, letting the paper hang over two sides.
In a bowl, combine the flour and icing sugar. Add the butter and combine until the dough forms. Press onto the bottom of the prepared dish.
Bake for 10 minutes or until light golden brown.
Meanwhile, in a saucepan, bring the condensed milk and butter to a boil, then simmer for 5 minutes, whisking constantly. Pour the caramel over the warm shortbread.
Bake for 15 minutes. Let cool completely on a wire rack.
Place the chocolate and cream in a bowl. Microwave for 1 minute or until heated through. Using a spatula, stir until the chocolate is melted and the ganache is smooth. Spread over the cooled caramel. Immediately sprinkle with the pretzels. Let set for 1 hour at room temperature.
Cut into squares. The millionaire shortbread with pretzels will keep for 1 week in an airtight container at room temperature.