Preheat your oven to 180°C (350°F) and grease a 20 cm (8-inch) round cake tin.
In a large mixing bowl, combine 200 grams of crushed biscuits (such as Marie or Digestive) with 100 grams of melted unsalted butter. Mix well until the biscuit crumbs are evenly coated with butter.
Press the biscuit mixture into the bottom of the prepared cake tin, creating an even layer. Refrigerate for 15 minutes to set the base.
In another mixing bowl, whisk together 200 grams of all-purpose flour, 1 teaspoon of baking powder, and a pinch of salt. Set aside.
In a separate bowl, cream together 100 grams of softened unsalted butter and 150 grams of granulated sugar until light and fluffy. Add 2 large eggs, one at a time, mixing well after each addition. Stir in 1 teaspoon of vanilla extract.
Gradually add the flour mixture to the butter mixture, alternating with 120 ml of milk, beginning and ending with the flour mixture. Mix until just combined.
Pour the cake batter over the biscuit base in the cake tin, spreading it evenly.
Bake for 30-35 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow the cake to cool in the tin for 10 minutes before transferring it to a wire rack to cool completely.