Take a medium-size bowl and add to it, plain flour, oats, brown sugar, granulated sugar, and butter. Mix them gently with soft hands.
Rub with ur palm softly to make a lump-free mixture. Add baking powder, cinnamon powder, salt, and vanilla essence to the mixture and mix it well. Put the crumble mixture in an airtight container and refrigerate it for 1 hr.
Preheat the oven to 180 degrees Celcius.
To make Berries filling, take out frozen berries and keep them aside for 5 minutes in a medium-size bowl. Now after 5 minutes add sunflower seeds, plain flour, brown sugar,salt, lemon juice, and vanilla essence to the berries. Mix them well with the spatula.
Now take your 8 by 8-inch baking dish and drop your berries filling in it, and lined it evenly with the help of a spatula.
On top of the berries filling evenly spread the crumble crust with your hands to form and half-inch thick layer. Press it gently to form an even layer, with the help of a spatula.
Put your baking dish in a preheated oven.
Bake it at 180 degrees for 30 to 40 minutes.
Keep it out for 5 to 6 minutes on an iron rack.
Serve it hot with ice cream.