Prepare the Cake Batter: Preheat your oven to 180°C (160°C fan/Gas Mark 4). Grease and flour your 9x13 inch baking tin, or line with parchment paper, leaving an overhang on the longer sides to help lift the cake later. In a large bowl, whisk together the plain flour, caster sugar, cocoa powder, bicarbonate of soda, baking powder, and salt until they are uniformly combined and the cocoa powder is fully incorporated, showing no dry clumps.

Combine Wet Ingredients: In a separate medium bowl, lightly whisk the eggs, then add the buttermilk, vegetable oil, and vanilla extract. Whisk these wet ingredients together until they are well combined and the mixture looks smooth and pale yellow.

Mix Dry and Wet Ingredients: Gradually add the wet ingredients to the dry ingredients, mixing on low speed with a stand mixer or hand mixer until just combined. The batter will be thick and slightly lumpy, but don't overmix. Carefully pour in the hot water or hot coffee, mixing until the batter is smooth and pourable. It should be a rich, dark brown colour and fairly thin.

Bake the Cake: Pour the chocolate cake batter evenly into your prepared baking tin. Bake for 35-40 minutes, or until a skewer inserted into the centre comes out clean. The top of the cake should spring back lightly when touched, and your kitchen will be filled with a wonderful, deep chocolate aroma. Transfer the cake in its tin to a wire cooling rack and let it cool for about 15-20 minutes.

Poke and Infuse: While the cake is still warm, use the handle of a wooden spoon or a round skewer to poke holes all over the top of the cake, ensuring they go almost all the way to the bottom. Aim for about 1-inch (2.5 cm) spacing between holes. Gently warm your caramel sauce slightly if it's too thick, then pour it slowly and evenly over the warm cake, making sure to fill all the poked holes. Watch as the glossy, golden caramel sinks into the dark chocolate cake, creating visible streaks in the holes. Allow the cake to cool completely at room temperature, then chill in the refrigerator for at least 30 minutes to allow the caramel to set further.

Prepare Frosting and Finish: In a clean, chilled bowl, whip the double cream with the icing sugar and vanilla extract until stiff peaks form and the cream is light, airy, and holds its shape. Spread this creamy frosting evenly over the chilled, caramel-infused cake. Finally, generously sprinkle the toffee bits or crushed toffee bars all over the top. The cake is now ready to slice and serve, revealing its beautiful caramel-filled interior.