A touch of egg is the simple, secret ingredient in this luscious tart's filling. Just a little gives the combination of chocolate and cream a sliceable, fudgy consistency. The crumbly cocoa-laced crust can be pressed right into a fluted pan, no rolling pin required. Swap out espresso beans for toasted nuts, chopped brittle, granola, or crushed peppermint candy. Just don't eliminate the sea salt; it adds a bright, irreplaceable contrast to the decadent filling.