Preheat oven to 350F; line bottom of 9 springform pan with non-stick foil or parchment paper, and wrap outside of pan with a layer of heavy-duty aluminim foil.
Place chocolate in large bowl; set aside, In medium saucepan bring butter, sugar, and espresso to a boil, stirring to dissolve sugar; once melted and blended, add to chocolate, whisking until smooth; cool slightly.
Lightly beat eggs together; whisk in some of chocolate mixture, then add egg mixture into remainder of chocolate mixture, whisking until smooth.
Pour batter into springform pan, place pan in a large roasting pan, pouring enough hot water to come halfway up the sides of the pan.
Bake for approximately one hour, or until a cake tester comes out with a few moist crumbs attached; remove cake from pan and chill overnight.
Cut around sides of pan with butter knife to loosen cake.