First, melt the butter in a microwave-safe bowl.
Stir in the sweetened condensed milk, cocoa powder, and vanilla extract until smooth and chocolatey.
Chop the tea biscuits into small bite-sized pieces and add them to the bowl.
Chop the California walnuts into small pieces and mix them in with the biscuit mixture.
Stir everything together until fully combined and evenly coated.
Transfer the mixture onto a large sheet of plastic wrap.
Shape the mixture into a log, then roll the plastic wrap tightly around it, twisting the ends to seal.
Place the wrapped log into the refrigerator for at least 2 hours, or into the freezer for about 45 minutes, until firm.
Unwrap the chilled or frozen log and, if desired, dust with powdered sugar.
Slice into rounds and serve.