Preheat oven to 350 degrees. Set aside an 8x8 casserole dish.
In a large mixing bowl, add in the brown sugar, flour, cardamom, and cinnamon. Mix with a spoon until the spices and flour are well combined into the sugar.
Add in the blackberries and carefully toss in with the sugar mixture. You don't want to smash your blackberries.
Add in the lemon juice and vanilla and gently toss the berries again.
Add the filling into the bottom of the casserole dish.
Using the same bowl that is now empty, add in the flour, sugar, and baking powder. Stir until everything is combined.
Remove the butter from the fridge and grate into the flour mixture using a cheese grater. I grate some and then cover the shreds of butter with flour so it doesn't lump back together. Keep grating some and covering with flour until all the butter is grated in.
Mix the butter into the flour. I used my hands to mix it.
Add in all of the heavy cream at once. Let it soak up for a minute and then mix. You want to mix very gently and just until the flour is incorporated. The more you mix the topping, the less of a light and fluffy topping you'll have.
Add the topping into the casserole dish over the berries. Spread out the pieces to cover all the berries.
Sprinkle cinnamon and sugar on the top to garnish.
Place in the oven and bake for 1 hour and 10 minutes or until the topping is baked through. There shouldn't be any doughy-like texture in the cobbler topping.