1. Preheat the oven to 400° F. Lightly butter 4 (6-ounce) ramekins. Place on a baking sheet.2. Melt the butter, chocolate chips, and chopped dark chocolate together over low heat, stirring often until smooth.3. In a bowl bowl, whisk the eggs and egg yolks for 1 minute until until pale yellow. Add the sugar, flour, espresso, and salt. Whisk the melted chocolate into the eggs. Add the vanilla. 4. Divide the batter between the prepared ramekins. Insert 1 chocolate square into each cake. Bake until the tops are just set, 18 to 20 minutes. Serve warm with chocolate shavings and lightly whipped cream.