Cognac or brandy1/4 cup (dry white wine or sherry can be substituted)
2tbspchopped fresh herbslike thyme, parsley, chervil or tarragon, (optional)
Heat the 4 tbsp of butter in a large saute pan, when the butter is melted, add the porcini season lightly with salt and pepper and cook until lightly browned.
Move the porcini to the side of the pan and then add the shallots. Cook on medium heat until the shallots are translucent and no longer raw. Next add the cognac and flambé. Cook until the flame dies and the pan is nearly dry, then transfer the mixture to a food processor and puree to a fine paste.
Allow the porcini mixture to cool to room temperature in the food processor, then begin adding the cold butter a few chunks at a time, adding more butter when the last butter you added becomes absorbed into the mixture. The mixture should become light and creamy. When the entire pound of butter has been absorbed, the finished product should be very shiny and thick. Taste the butter for salt and pepper and adjust as needed.
Add the herbs if using and then transfer to another container and refrigerate until needed. Or freeze in logs, wrapped in plastic wrap, ice cube trays, in a plastic freezer bag, etc.
It goes without saying you could make this with other boletes, but porcini have more flavor fresh than most I've tried, so they're my first choice.