Lightly grease an 8- x 8-inch collapsible pan with butter. Set aside.
Heat cream and butter over medium-high heat to 205°F (check with a digital thermometer). It will be steaming and starting to bubble. Remove from heat.
Add the chopped chocolate. Cover and let sit 3 to 4 minutes
Stir until smooth. Add Kahlua & espresso powder, vanilla, and nuts (if using).
Spread in prepared pan and smooth using a small offset spatula. Let the pan sit for 4 hours at room temperature, then briefly refrigerate for 1 to 2 hours or until firm.
Cut the fudge into 1-inch square pieces with a knife warmed in boiling water and wiped dry. Cut in one stroke, if possible.
Store the fudge in an airtight container in the refrigerator. The fudge will last for 4 weeks.
To freeze, wrap the fudge first in wax paper, then in plastic wrap. Put in ziplock bag. It will last for several months.
I like using dark chocolate in fudge...it contrasts nicely with the coffee flavor of the Kahlua. You can substitute semi-sweet chocolate or even white chocolate if you really want to highlight the Kahlua. Just use less of the espresso powder, or leave it out altogether. The lighter you go, the sweeter your Kahlua Fudge will be.You can use chopped nuts, like pecans, walnuts, or pistachios. Lightly toasting the pecans really brings out their buttery flavor, but I'd leave the walnuts or pistachios raw.If you don't have an 8- x 8-inch collapsible pan, you can use an 8- x 8-inch baking pan. Line the pan with parchment paper or foil, allowing the excess paper to overlap. Grease the parchment or foil with butter.This fudge is made like ganache, thus insuring that the chocolate (especially white chocolate) doesn't burn.