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Lightly grease a small cake ring or springform pan with 1/2 teaspoon butter and set it aside. If you are using a cake ring, place it on a baking sheet lined with parchment paper. In a large bowl, crumble the biscuits into small almond sized pieces. Use about 1 cup of biscuit crumbs and set the bowl aside. Using more or less crumbs in larger or smaller chunks will change the texture and consistency of your cake. You can experiment with this if you'd like. In a medium bowl, mix 1/2 cup butter and 1/2 cup sugar until combined. Stir the mixture with a large spoon or rubber spatula until your mixture is a light lemon color. You may need to let your butter come to room temperature or heat it slightly to allow it to mix with the sugar. . You can melt chocolate in a pan on the stove over low heat or pop it in the microwave in increments of 15 seconds to melt it. Only melt half of your dark chocolate. You will need the other half to frost your cake later.  Do not allow your chocolate to burn. Remove your chocolate from the heat once it has melted. Stir by hand with a large spoon or rubber spatula. Slowly pour the butter-sugar mixture into the melted chocolate. Add the egg and continue stirring. Pour in the biscuit pieces and fold them into the mixture. Continue folding in the biscuit pieces to coat them completely with chocolate. Pour or spoon the mixture into your cake ring. Lay the mixture as evenly as possible to prevent any gaps from forming at the bottom. For a denser cake, use your hands in plastic sandwich bags or use disposable plastic cooking gloves to press your mixture down into the pan.  You don't need to press very hard, just lightly pack it in. Pressing your cake down will help you achieve a smooth, even surface. Move the trap or spring-form pan into the refrigerator and let it chill for at least 3 hours. Chilling your cake is necessary to achieve a smooth thick texture that holds together when slicing. Take your cake out of the refrigerator and remove it from the ring or springform pan. Once your cake has chilled, it will be firm enough to transfer to a cooling rack. You can flip your cake upside down onto the cooling rack. This may be easier. Melt the second half of your dark chocolate (1/2 cup) and pour it over the cake, smoothing the top and sides with a butter knife or rubber spatula. Allow the icing to set at room temperature. Melt 1/4 cup of milk chocolate and drizzle it over the top of your cake for a decorative touch. Transfer your finished cake to a cake dish or platter and serve. Run a butter knife between the bottom of the cake and the cooling rack to gently pry the cake up if it has begun to stick to the rack.
Prepare your cake dish. Crumble the biscuits. Cream together butter and sugar. Melt the chocolate Mix everything together. Make your cake. Chill your cake. Remove your cake from the cake ring. Frost your cake. Enjoy!