Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish or line it with parchment paper.
In a large mixing bowl, combine the flour, granulated sugar, brown sugar, baking powder, baking soda, salt, and cinnamon.
In another bowl, whisk together the buttermilk, oil, eggs, and vanilla.
Pour the wet ingredients into the dry ingredients and stir until just combined. Don’t overmix—nobody likes a tough cake.
Pour the batter into the prepared dish and smooth the top with a spatula.
In a small bowl, mix together the crumb topping ingredients: flour, brown sugar, cinnamon, and softened butter. Use your fingers or a fork to create a crumbly texture.
Sprinkle the crumb topping evenly over the cake batter.
Bake for about 40 minutes, or until a toothpick inserted into the center comes out clean.
Let it cool slightly before slicing. Then pour yourself a cup of coffee and try not to eat half the pan.