In a large mixing bowl, combine the sweetened condensed milk, melted butter, and cornflour. Stir with a spatula or use your hands until a smooth dough forms.

Divide the dough and roll it into logs about 1½ inches (4 cm) in diameter. Wrap each log tightly in plastic wrap and refrigerate until firm, at least 4 hours. (The dough can be stored in the fridge for up to 1 week.)

Preheat the oven to 180°C/ 350°F/ gas mark 4. Line a baking sheet with parchment paper.

Unwrap the chilled dough logs and slice them into ½-inch (1 cm) thick rounds. Place them 1 inch apart on the prepared baking sheet.

Bake for 15-20 minutes, or until the cookies are lightly golden on the bottom. Allow them to cool completely on a wire rack.

Chocolate Coating: Melt the chocolate using a double boiler or in the microwave at medium power, stirring every 30 seconds until smooth.

Dip each cooled cookie into the melted chocolate and place it on parchment paper. Allow the chocolate to set at room temperature or chill in the fridge for about 5 minutes.