Preheat the oven: Preheat your oven to 350°F (175°C). Grease an 8×8-inch baking dish or line it with parchment paper.

1. Prepare dry ingredients: In a medium-sized bowl, combine the oats, baking powder, salt, and cinnamon. Stir to combine and set aside.
2. Prepare wet ingredients: In another bowl, whisk together the peanut butter, maple syrup (or honey), milk, eggs, and vanilla extract until smooth and well combined.
3. Combine wet and dry ingredients: Add the wet ingredients to the dry oat mixture and stir until everything is well combined.
4. Assemble: Pour half of the oat mixture into the prepared baking dish and spread it out evenly. Spoon the fruit jam or jelly over the oat mixture, spreading it out in an even layer. Then, pour the remaining oatmeal mixture on top, spreading it out evenly over the jam layer.
5. Swirl the jam: Use a spoon to gently swirl the jam into the oatmeal, creating a marbled effect.
6. Bake: Bake in the preheated oven for 30-35 minutes, or until the oatmeal is set and golden on top.
7. Cool and serve: Remove from the oven and let cool for 5-10 minutes. Top with chopped peanuts or more peanut butter, if desired, and serve warm.