1.Using a standing mixer with a paddle attachment beat butter for 10 minutes on high speed until pale and fluffy. Transfer to a bowl and set aside. Wash mixer bowl and dry.
2.Add egg yolks and caster sugar into mixer bowl, then using the whisk attachment beat for 10 minutes on high speed.
3.Lower your mixer speed and add in flour and milk powder. Add vanilla extract and beaten butter. Mix until fully incorporated.
4.Preheat oven to 350 degrees F. Spray three loaf pans (8"X4") with cooking spray then line with parchment paper, set aside.
5.In a small bowl, mix cocoa powder and 1 tablespoon of water until well combined. Divide the batter into thirds.
6.Place one third of batter into one loaf pan. Place another one third of batter into the second loaf pan. The last third of batter left in the bowl should be mixed with the cocoa mixture then place in the last loaf pan.
7.Bake cake for 20 minutes. Take the cake out of the pan and take the parchment paper off then add to a cooling rack to cool completely. Place one piece of the yellow cake on a plate, spread with 1 tablespoon of strawberry jam. Place chocolate layer on top then spread with another 1 tablespoon of strawberry jam. Top with last layer of cake. Refrigerate for 1 hour prior to serving.