Place the digestive biscuits in a food processor and pulse until you get fine crumbs. Or place them in a sealed ziplock bag and crush with a rolling pin.
In a mixing bowl, stir together the biscuit crumbs and melted butter until the mixture resembles damp sand.
Pour the mixture into the base of an 8in x 8in square dish or pie dish. Use the bottom of a glass or measuring cup to firmly press the crumbs down into an even layer, going slightly up the sides if desired.
Bake at 175°C (350°F) for 8–10 minutes, then let cool.