Preheat your oven to 350°F. Spray a 9x13 baking dish with cooking spray. Since this cake stays in the pan for serving, you don't need to line it with parchment — just a good coating of spray will do.
Prepare the buttermilk mixture. Measure out 2 cups of buttermilk and whisk in 1 teaspoon of baking powder. Set it aside — this is going to be our egg replacement and the key to keeping this cake moist.
Combine the dry ingredients. In a large bowl, stir together 4 cups of flour, 2 cups of sugar, 2 teaspoons of baking powder, and 1 teaspoon of salt. Use the spoon-and-level method for your flour so you don't end up with too much — that's what can make a cake dry and tough.
Grate in the butter. Take ¾ cup of cold butter straight from the fridge and grate it right into the dry ingredients using a hand grater. Then scoop some of the flour mixture over the top and stir it in. This coats the butter pieces and helps distribute them evenly. You'll see little bits of butter throughout, and that's exactly what you want — it's going to make the cake tender and flaky.
Add the buttermilk. Pour the buttermilk mixture into the dry ingredients and stir gently until everything is just combined. Do not overmix. A few small lumps are perfectly fine. Overmixing will make the cake tough, and we definitely don't want that. This is a thick batter — much thicker than a typical cake batter — and that's normal.
Spread the batter into your prepared baking dish. Use the back of your spoon or a spatula to gently even it out to the edges.
Make the streusel. In a separate bowl, combine 1 cup of brown sugar, 1 tablespoon of cinnamon, and 1½ sticks of grated butter. Mix it together well, breaking up any clumps of brown sugar. Divide this mixture in half.
Swirl half the streusel into the cake. Take about ¾ cup of cake batter out of the pan and stir it into one half of the streusel. Then drop spoonfuls of this mixture back onto the cake and use your spoon to push it down and swirl it through the batter. You want that cinnamon brown sugar goodness running all through the middle of this cake so every bite has some.
Make the crumb topping. Take the remaining half of the streusel and add 3 tablespoons of flour to it. Toss it together — the flour helps it separate into a nice crumbly topping instead of melting flat during baking. Sprinkle it evenly over the top of the cake, using your hands to break up any sticky spots.
Bake at 350°F for 45 to 60 minutes. Start checking at 45 minutes with a toothpick — when it comes out clean, the cake is done. Every oven is different, so keep an eye on yours. The streusel on top should be golden brown and just a little bit crispy.
Let it rest. Give the cake about 30–40 minutes to cool before slicing. It's wonderful served warm, and it holds its texture beautifully even as it cools to room temperature.