Preheat oven to 350° F/175 C. Grease an 8”/20cm square baking pan or line the bottom with parchment.
In a small bowl, combine 1/4 cup/50g of light brown sugar, 6 tbsp/45g of all purpose flour, 1/2 tsp/1g ground cinnamon, and 2tbsp/30ml neutral oil. Stir to evenly distribute the oil.
Add water a little bit at a time and stir or crumble with your fingertips into small clumps. Set aside.
Combine the sugar, oil, apple sauce, vanilla, and oat milk in a large bowl. Whisk until smooth.
Add the flour, baking powder, cinnamon, nutmeg, and salt. Stir until smooth.
Fold in the blackberries until evenly coated.
Scrape into prepared pan and level out.
Crumble the streusel on top so that it is in contact with the batter.
Bake for 40-45 minutes, until golden brown on the edges, set in the center and skewer inserted comes out clean.
Allow to cool in the pan for at least 30 minutes before transferring to a cooling rack.