Mix the melted butter with the crushed cookies until it resembles wet sand. Press this mixture into a lined and greased 7-inch springform pan. Chill in the fridge while you make the brownie.

Preheat your oven to 350°F. In a saucepan, melt the butter and sugar on low heat, then add the milk. Turn off the heat, add the chocolate, and stir until smooth. Mix in the yogurt. In a large bowl, sift together the flour and cocoa powder. Combine the wet ingredients with the dry ones. Pour the batter over the cookie base and bake for 20 minutes, or until the top is set. Let it cool completely.

Heat the custard until boiling and add the chocolate. Let it melt, then stir until smooth. Pour this over the cooled brownie layer. Cover with plastic wrap to prevent a skin from forming, and refrigerate overnight to set.

The next day, top with whipped cream and sprinkle with chocolate shavings.