Line one baking sheet with parchment paper or a silicone baking mat.
Once doubled in size, lightly sprinkle the dough with flour and punch out the air from the dough. I recommend not re-kneading it because this will tighten the gluten making it difficult to roll out but just be sure to smoosh out all of the air.
Lightly flour your work surface and pat the dough into a skinny rectangle. Gently roll the dough out to approximately 30 cm x 50 cm. I recommend rolling the short end closest to you a little thinner so the center isn't too thick.
Spread 30 grams of the very soft butter over the entire surface of the dough. I recommend doing this with a pastry brush or an offset spatula.
Spoon the plum butter over the thin butter layer and use an offset spatula or a rubber spatula to spread it over the whole surface of the dough, all the way to the edges.
Starting with the short side closest to you, roll the dough up tight but not too tight that all of the jam squeezes out.
Use a knife and ruler to mark the log into 6 even slices.
Use the same sharp knife to cut the roll into slices. Lay each of the slices onto the lined baking sheet. Tuck about a 1/2" of the end of the swirl under each roll.
Gently press down on top of each slice, it will be a little messy, but this helps to give them their characteristic large swirl.
Tuck about a 1/2" of the end of the swirl under each roll.
Gently press down on top of each slice, it will be a little messy, but this helps to give them their characteristic large swirl.
Press the center of the swirl down to make a little well. Spoon about 2-3 tablespoons of the vanilla custard into the center well.
Leave the rolls to proof for about 20 minutes, they should look like they have grown slightly and are a little puffy.
Once proofed, bake the rolls for 15 minutes in the preheated oven. Then rotate the pan 180 degrees and bake for another 15 minutes until golden brown.