Make the Swiss roll

1. Preheat the oven to 180°C. Grease the Swiss roll pan 15" x 9" with butter and line with a sheet of parchment paper.
2. Prepare the walnuts Toast 150g walnuts in a cast iron pan until they turn golden brown or you can smell the nutty flavour. You can also bake them in the oven at 200°C for about 5-10 mins.
3. Cool the toasted walnuts on foil for about an hour.
4. Pour the cooled toasted walnuts into a food processor and add ½ tsp baking powder. Grind them until sandy-texture; stop the processor just before they turn into a paste.
5. Prepare the eggs Separate egg yolks and egg whites into two bowls. DO NOT let any egg yolk drop into the egg whites.
6. Beat the egg whites Pour the whites into a stand mixer with a whisk attachment. Whisk the whites at a low speed for a few minutes. When you see some air bubbles are formed at the edge of the whites, add ¼ tsp of cream of tartar to help stabilise the whipped egg whites.
7. After a couple of minutes, when you see a layer of bubbles has formed, add 75g caster sugar, spoonful by spoonful to ensure the sugar is fully dissolved. After all the sugar is added, increase the speed to high and keep whisking until a stiff peak forms.
   
8. Beat the egg yolks Put 6 egg yolks into a large mixing bowl and add the remaining 75g caster sugar. Use a hand-held or stand mixer with a whisk attachment and start beating the yolks at medium speed. Beat the yolks and sugar mixture for around 4 minutes or until the mixture becomes thick in texture and pale in colour.
   
9. Add the ground walnut mixture into the egg yolk mixture. Gently fold and mix them together using a spatula until they are well combined.
10. Mix the mixture Scoop a little bit of egg white into the walnut mixture to loosen the texture first. Then fold the egg whites into the walnut mixture in 3 portions. Do not forget to scrape the bottom and the sides of the bowl to make sure everything is well mixed. Be patient at this stage because you do not want to knock any air out by whisking. Keep folding and mixing until you cannot see any blobs of egg white.
11. Carefully pour the mixture into the prepared tin. Spread it evenly using an offset spatula.
12. Bake the cake Bake the cake in a preheated oven (180°C) for about 10-12 mins or until the sponge is golden brown and springy to the touch.
13. While the cake is baking, prepare a sheet of parchment paper which is slightly larger than the cake on a flat surface.
14. After 12 mins, the cake is done. Make sure the sides of the cake are not sticking onto the baking pan. Be careful, the cake and the pan are very hot.
15. Lift up the cake and flip it onto the prepared parchment paper. Peel off the baked parchment paper carefully.
16. While the cake is still warm, start rolling it from the short side. Use the parchment paper to help roll the cake tightly. Place the seam down and let it cool completely on a wire rack.

Make the coffee cream filling

1. Prepare the mixture Melt 250g unsalted butter in a saucepan at low to medium heat. When the butter comes to the boil, turn off the heat and let it cool down a bit.
2. Put 400g sifted icing sugar into a stand mixer with a whisk attachment. Pour the melted butter into the icing sugar slowly. If you have white residue from the melted butter, leave it in the pan.
3. Keep whisking the mixture for around 5-7 mins or until it becomes smooth in texture. Stop the mixer once or twice in between to scrape down the sides and the bottom of the bowl to make sure everything is well combined.
4. Whisk the cream and the coffee After around 5-7 mins whisking, if your bowl is still warm, keep whisking it until it becomes a bit cooler (not warm to touch) before adding the brewed coffee and the chilled cream.
5. Keep the mixer at low or medium speed, then add 6 tbsp chilled double cream before then adding 6 tbsp brewed coffee. Initially, the mixture will curdle.
6. Turn the speed to high for 2-3 minutes and keep whisking, until the mix acquires a smooth texture.
7. Cover the whipped coffee cream with a lid or cling film. Keep it cool in the fridge and take it out just before assembling the cake.

Assemble the cake

1. Gently unroll the cooled Swiss roll. Scoop a few generous blobs of chilled coffee cream onto the cake and spread it evenly using an offset spatula. Make sure there is enough cream inside the curved areas. Leave around 1 centimetre at one end. Don't worry about the sides, they will be trimmed to become the ends of the cake.
2. Without parchment paper, start rolling the cake back into a tight log. Make sure the seam is facing downwards.
3. Use a sharp knife to trim both sides to make the roll look tidy.
4. Ready a serving plate and place the Swiss roll onto it using a cake lifter or a spatula.

Decorate the Swiss roll

1. Pipe the stars Cut a hole at the end of a piping bag and insert an open star piping tip. Use a tall glass to hold the bag while loading the remaining coffee cream into it. Secure it with a clip at the top.
2. Hold the piping bag straight and apply gentle pressure. Pipe a star in the middle of the roll. Then pipe three more each in front of it and behind it.
3. Sprinkle the Walnuts Finally, sprinkle some chopped toasted walnuts on the stars. Slice cake and enjoy!