Place the butter in a pot over medium heat. Let it melt and start to sizzle. You'll first see foam on top, then it will go away and finally, you will see brown specks in the butter and it will stop sizzling.
Once the butter is ready, add the brown sugar, the apples, cinnamon, salt and mix.
After 10-20 minutes of minutes of mixing over medium heat, the mix will thicken and look like a caramel sauce. Please be careful, this mix is extremely hot and it's best if there are no kids close by. It should look like this, coating the apples:
Once ready, pour out the mix into a bowl. We need it to cool down enough so that it doesn't burn if you touch the outside of the bowl (please don't put your finger in the mix). It should cool down enough so that it's still hot, but not enough to burn you. As it cools down you will notice that it will thicken.
Add the egg and quickly mix with a spatula.
Add the flour and mix.
Prepare a 20cm / 8in square pan with baking paper. You can stick it in place using a bit of oil, spread with a piece of kitchen paper.
Pour the batter into the pan and spread it out.
Bake the blondies in a preheated oven at 180°C/350°F for 15-20 or until a bit of cooked but gooey dough comes out if you stick a knife or skewer at the centre.
Once cold, unstick the edges by running a thin knife and lift it off using the baking paper. Cut the blondies in 4-5cm squares.