Heat milk and cookie butter in a medium saucepan over low heat until hot.
Meanwhile, in a medium glass bowl, whisk together brown sugar, egg yolks, and corn starch.
Once milk mixture is hot, remove from oven. While whisking constantly, add about ¼ cup of milk mixture to the brown sugar and egg yolk mixture. Once combined, slowing add in remaining Speculoos milk, whisking to prevent eggs from getting too hot. Pour mixture back into saucepan, and return to a low to low medium heat. Cook, whisking, until mixture is thickened and almost pudding like consistency (it will continue to thicken as it cools). Remove from heat and whisk in butter and a pinch of salt. Strain mixture into a bowl. Cover bowl and place in the fridge to cool.
Preheat oven to 350 degrees F. Line two baking sheets with parchment paper.
Divide brioche dough into 12 evenly sized pieces, then roll into balls. Flatten each dough ball and shape into a thin circle. Fold edges over to create a rim around the edges.
Spoon pastry cream (about 2-3 tablespoons per pastry) into the center of the dough circles. Add some fanned out plum slices to the center or the pastries.
Stir granulated sugar and cinnamon together in a small bowl. Sprinkle tops of pastries with cinnamon sugar. Bake in preheated oven for about 18-22 minutes, until cooked and golden brown on the edges. Remove from oven and let cool before serving.