Preheat the oven to 170°C (with circulation fan 150°C).
For the pastry base, beat 150g of soft butter to a cream with the egg and 80g of sugar. Fold in 200g flour and knead to a flat pastry.
Grease a baking dish (approx. 30x30cm) with butter. Place the pastry in the dish, prick several times with a fork, and bake for 20 to 25 minutes in the pre-heated oven. For the toffee mixture, gently boil 50g butter, 50g sugar, a pinch of salt and Bonny Sweetened Condensed Milk for about 5 to 8 minutes until it becomes viscous, stirring constantly.
Finally, stir 100g chopped hazelnuts into the cooked toffee. Distribute the toffee mixture evenly over the pre-baked pastry, and continue to bake for about 15 minutes at the same temperature. Allow it to cool in the dish in the refrigerator. Then cut into approx. 2x2cm squares.
It is important to grease the baking tin generously and evenly with spreadable butter or margarine using a brush before baking. To enable the baked good to be released more easily from the tin after baking, sprinkle the tin with flour or chopped nuts.