When you take the brownies from the oven the centre will look undone, but it keeps cooking when resting. And if you leave them overnight in the fridge, the next day they will be even more consistent.
This recipe also works well with 3 eggs, 100g butter, 65g to 125g of flour, 45g to 65g cocoa. So, don't worry if you don't have enough of any of the ingredients, but make sure not to change more than one amount at a time!
You can also stir in 120g of nuts, or 60g of nuts and 80g chocolate chips, or only chocolate chips! Or you can sprinkle some chopped nuts or pistachio nuts (tastes great!) on top before microwaving.