Preheat the oven to 180 C (350 F). Line a 20cm (8-inch) square baking tin with baking paper. 

Place the flour, sugar and butter into a large bowl. Use your fingertips to rub the butter into the dry ingredients until you get a mealy texture. Add the crushed pretzels and mix through into the mixture. Tip into the lined baking tin, spread it out in an even layer, then press down to make a smooth, even base. Bake for 15-20 minutes - until the top start to colour golden. Remove from the oven and set aside. 

While the base is baking, combine the caramel ingredients (sweetened condensed milk, butter, sugar, malted milk powder) in a medium pot. Place over a low heat and stir constantly (I like to use a silicone spatula) until the butter has melted. Turn the heat up and bring to the boil - keep stirring and cooking until the mixture has thickened and darkened slightly. 

Pour the warm caramel over the cooked shortbread base and spread out in an even layer. Set aside to cool completely.

Once the caramel has cooled, pour the melted chocolate over it and spread out in an even layer. Decorate with the pretzels (I managed to fit a grid of 5 x 6 on but it'll depend on the size of your pretzels). Leave to cool and set before cutting into squares with a hot knife.