In a pot, boil the cooking cream, sugar, and Thai tea leaves for 5 minutes, till it bubbles. Then remove from heat. 

Strain liquid to remove the tea leaves. 

Divide the Thai tea cream, 50 g for batter and 250g for the filling. 

Place the 250g of Thai tea cream in the fridge till well chilled, or about 2 hours.

In a bowl, whip 5 egg yolks with 50g of sugar until pale and fluffy. Set aside. 

In another bowl, whip the 5 egg whites with 70g of sugar until stiff peaks form. 

Add the egg yolk batter into the egg white batter and gently fold till incorporated, forming an off-white color.

Sift the all-purpose flour and baking powder together. Add into the batter and gently fold till incorporated. 

In a separate bowl, add the 50g of Thai tea cream and melted butter together. Then add two dollops of batter to the Thai tea cream liquid mixture to temper it, preventing it from splitting. Add to batter and fold in.

Preheat the oven to 180°C 

Line the base of a 10-by-15-inch baking pan with parchment paper.

Pour the batter and spread evenly. Bake for 20 minutes, then side aside and leave the cake to cool.

Remove the 250g of Thai tea cream from the fridge once it has been chilled for 2 hours.

Spread Thai tea cream on the cake, leaving ½ inch on one side of the cake.

Roll the cake gently. 

Place in the fridge before 15 minutes to set.