Preheat oven to 350 degrees F. Line a backing sheet with parchment paper and set aside.
Process the oats and chia seeds into a food processor until you get a flour-like mixture.
Transfer them to a bowl, and add the baking powder. Set aside.
In a blender add all other ingredients and blend until smooth. Pour this mixture into the oats mixture and stir well to combine. Cover with plastic foil and leave in the fridge for 30 minutes.
Once you take the mixture out of the fridge it should be very thick. Place it on some baking paper, and put some paper on top too, then pull out a thin crust with a rolling pin (whenever I make a cake crust with a rolling pin, I put paper on top so that the rolling pin doesn't stick, try it, it's very practical).
With your hand make circular biscuit-shaped cookies (Around 2 tablespoons of the mixture for 1 biscuit) and place them on the baking sheet.
Using a small mold, (in my case with a small cup) take out small circular shapes and make a hole in the middle for half of the biscuits. So, if you got 20 biscuits, do this with 10 of the biscuits.
Bake your biscuits for about 20 minutes. Set aside to cool.
Take one whole biscuit, spread some plum jam on top and cover with a biscuit with a hole. Repeat with the rest of the biscuits. Enjoy!