American fudge slice

1. Preheat oven to slow, 160°C. Lightly grease and line an 18 x 28cm slice pan.
   
2. Using an electric mixer, beat butter and sugar in a medium bowl, until pale and creamy. Add egg, beating well.
   
3. Sift flour and cocoa together. Fold into creamed mixture. Add remaining ingredients and mix until just combined.
   
4. Press into prepared pan. Bake for 30 minutes. Cut into squares while still warm. Store in an airtight container.