Crush the biscuits finely. You can use a food processor or place them in a bag and crush them with a rolling pin.
Melt the butter in a small saucepan or microwave until completely liquid.
Combine the crushed biscuits with the melted butter in a bowl and mix well until everything is evenly coated.
Press this mixture firmly into the bottom of your 15 cm diameter cake form. Use the bottom of a glass to make the base even and compact.