Preheat oven to 375 F (190 C) for conventional oven, 350 F (180 C) for convection oven
I use half-sheet pan, which measures 13 x 18 inches (inner size is about 17 x 12 inches). You can also use two quarter-sheet pans 9 x 13. Brush them with some oil and line the pan with parchment paper on the bottom
Prepare the cake batter:
Combine the cake flour, milk powder, spekoek spices (if using) in a bowl and set aside
Put the softened butter and condensed milk in a mixing bowl fitted with a flat beater. Beat on high speed (speed 8 on KA) until pale and creamy, about 3-4 minutes. If you only have one mixing bowl, transfer the butter out into a bowl. Wash the mixing bowl clean and get ready to beat the eggs and sugar
Place room-temperature eggs and sugar in a mixing bowl. Change to a whisk attachment. Start with a medium speed and then continue to go up to high speed and continue to whip the egg and sugar and they are thick, pale, and creamy, about 8 minutes, depending on your mixer too. If you draw a figure 8 using the whisk, it won't disappear immediately. This is what we call a ribbon stage
Add the butter mixture into the yolk batter in 3 batches, mixing on the lowest speed. Sift in the flour mixture and whisk on low speed until just combined. Don't overdo it. The batter is thick
Spread the batter on the pan evenly using an offset spatula. Lay the flatten prune pieces on top of the batter
Baking:
Bake on the middle rack for 5 minutes. If you use two pans, you can bake side by side if the oven fits. Otherwise you may need to bake one tray at a time. If your oven can control upper and lower heat, turn on the upper heat only and bake for another 3 minutes or until they are golden brown and the surface is dry to touch
If you oven is like mine, without control of upper or lower heat, turn on the broiler on low and broil for 1-2 minutes or until the surface is golden brown. Do not walk away during this time as the cake can be easily burnt
Assembling (if baking in one large pan):
Remove from the oven. Place a piece of parchment paper on top and flip the cake over. Spread a thin layer of condensed milk on the surface and start rolling from the short side while the cake is still warm. Tighten the roll by tucking the parchment paper on top with a bench scraper while pulling the parchment paper at the bottom at the same time. Keep it rolled up to set the shape.
Assembling (if baking in two pans):
Remove from the oven. Place a piece of parchment paper on top and flip the cake over. Repeat with the other cake
Connect the end of the two cakes together on the short side so they are longer now. Spread a thin layer of condensed milk on the surface and start rolling from the short side while the cake is still warm. Tighten the roll by tucking the parchment paper on top with a bench scraper while pulling the parchment paper at the bottom at the same time. Keep it rolled up to set the shape.
Age the cake for 24 hours:
I highly recommend not serving it right away. The cake feels dry right after it bakes. The flavor and the fat have not had a chance to be fully distributed yet. The cake tastes better and better as time goes. It tastes moist, rich, and dense in a good way after that