Start your baking adventure by preheating your oven to 180°C (350°F). This is crucial. A well-preheated oven ensures that your cake rises beautifully.

Grab a large mixing bowl and add the 175g of unsalted butter along with the granulated sugar. Beat them together until light and fluffy. This can take around 3-5 minutes.

Next, crack in the 3 eggs, one at a time. Ensure each egg is fully incorporated before adding the next. Stir in the teaspoon of vanilla extract for that extra touch of flavor.

Now, combine the self-raising flour and baking powder in a separate bowl. Gradually fold this mixture into the wet ingredients. Add the chopped walnuts and dark chocolate chips into the mix. Finally, fold in 50ml of chilled espresso until everything is well combined.

Grease your baking tin with a bit of butter and pour the mixture into it. Use a spatula to smooth the top. Pop it into your preheated oven and let it bake for around 25-30 minutes. A skewer inserted in the center should come out clean when done.

As the cake bakes, whip up the filling. In another mixing bowl, beat together 65g of softened unsalted butter, 125g of confectioners' sugar, and 40ml of strong brewed espresso until smooth and creamy. This adds a lovely layer of flavor and moisture.

While the cake cools, prepare the icing. Mix 100g of confectioners' sugar with 20ml of strong brewed espresso in a bowl until you reach a gloopy consistency that can drizzle onto the cake.

Once the cake is baked and cooled, slice it in half horizontally. Spread the coffee filling on the bottom layer and place the top back on. Drizzle the icing over the cake, allowing it to cascade down the sides. Finish with some extra chopped walnuts on top for that gourmet feel.