According to the Edited Guide, some H2G2 researchers believe the 'chocolate bourbon cream' to be 'the single most perfect object known to man'. The Bourbon cream - familiarly known as just 'Bourbon', probably because each is stamped with the word Bourbon - was first produced in 1910 by the Bermondsey biscuit company.
That said, and supply sometimes failing to meet demand, here is a passable homemade version:
Nearly Bourbon Biscuits
INGREDIENTS - makes 14-16:
2/3 cup (75 g) icing sugar (confectioners sugar)
1/2 stick (2 oz) 50 g really high quality butter or margarine
1 tbsp high quality cocoa powder
1 tsp coffee essence or cold-brewed coffee syrup (desperate substitute: 1/2 tsp spoon instant coffee dissolved in 1 tsp water)
The Biscuit parts:
1. Heat the oven to warm, Gas 3, 160°C.
2. Prepare a baking sheet by covering the bottom with greased parchment paper.
3. Cream butter and sugar together very thoroughly.
4. Beat in the syrup.
5. Sift the flour, cocoa, and baking soda together.
6. Work the powder Mixture into the creamed mixture to make a stiff paste.
7. Knead well, and roll out on a lightly floured surface about 5 mm thick.
8. Cut into two 6 cm fingers.
9. Place on a greased baking sheet covered with greased greaseproof paper.
10. Dock the dough (pierce with a docking roller, skewer, or fork) every half-inch. (Helps prevent cracking during baking)
11. Bake for 15-20 minutes.
12. Let cool for one minute.
13. Cut, while still warm, into 28 to 32 identical, rectangular fingers.
The Filling part:
1. Sift the icing sugar.
2. Beat the fat until soft, add the sugar, cocoa, and coffee.
3. Beat until smooth.
Assembling the biscuits:
1. Carefully remove the fingers to a cutting board or work surface.
2. Keep the 'docked' sides out for a more attractive biscuit.
3. Slather the surface of half the fingers evenly with a layer of the filling.
4. Cover with an unslathered finger and press lightly.
5. If necessary, warm them up enough for the cream filling to soften so that they settle in and adhere.