If caramelisation – the liquefying and darkening of sugar – is my favourite scientific reaction, Maillard is my second. It occurs when browning butter, giving it its distinctly nutty, toasty smell and flavour profile. The browning becomes noticeable once the water has evaporated at 100°C, which is when the temperature can increase further and the lactose in the milk solids starts to brown – ‘Maillardisation’. The technique in this recipe works for any amount and type of dairy butter; just ensure that you increase the pan size if you increase the quantity of butter. This recipe, as an example, makes the quantity of brown butter required for Caramel Apple Pie Blondies. Excess butter can be stored in an ice-cube tray in the freezer for up to two months.