Begin by melting the dark chocolate spread and allowing it to cool slightly. While it cools, beat the softened butter together with the egg and sugar until you achieve a smooth, creamy consistency. Gradually incorporate the melted chocolate into the butter mixture, stirring continuously. Once fully combined, the chocolate buttercream is ready for use.

2. Assemble the Cake:
To assemble, dip each tea biscuit into cold espresso, ensuring not to soak it for too long. Place the dipped biscuits side by side in a serving dish of your choice (as this dish will be used for presentation). After arranging the first layer of biscuits, spread a generous layer of chocolate buttercream on top. Use a knife to gently spread the buttercream, taking care not to break the biscuits.

3. Layering:
Continue alternating layers of espresso-soaked biscuits and chocolate buttercream until you run out of ingredients. Finish the cake by completely covering the top layer of biscuits with the remaining buttercream.

4. Chill and Serve:
Once the cake is fully assembled, cover it with aluminum foil and refrigerate for at least 24 hours (48 hours is ideal for best results). Before serving, let the cake sit at room temperature for about 30 minutes to soften slightly.

Enjoy your indulgent Mocha Layer Cake with Chocolate Rum Cream Filling!