Preheat oven to 350°F and grease a 9-inch square pan.
Place toffee bars in the freezer for at least one hour before making. For base, combine flour, sugar and salt. Cut in 1/2 cup butter with fingers or a pastry cutter until a rough, crumbly texture and press into prepared pan. Dock base with a fork and bake 20 minutes, until edges start to brown slightly. Allow to cool.
For toffee layer, remove toffee bars from freezer and place in a sealable bag. Smash bars with a rolling pin or bottom of a pot to crush into small pieces. Pour toffee pieces into a small saucepot, reserving 1/2 cup of pieces for the top of the squares. Add 1/4 cup butter, corn syrup and water and melt over medium-low heat, stirring constantly. Once smooth, pour over shortbread base and spread to coat. Bake for 10 minutes, until toffee is bubbling. Allow to cool.
For chocolate topping, melt chocolate chips with remaining 3 Tbsp butter in a bowl over a pot of gently simmering water, stirring constantly (or in microwave on medium heat, stirring at 10-second intervals) until smooth. Spread over toffee layer and sprinkle with reserved hard toffee bits (you can smash them more if too large). Let chocolate set before slicing.