Heat your oven to 375°F and grease a 9-inch round cake pan with butter or cooking spray. Make sure to coat the sides well — this cake can stick if you're not thorough.
Combine the 2 cups flour, granulated sugar, baking powder, cinnamon, and salt in a large mixing bowl, whisking everything together until evenly distributed. The cinnamon should be completely mixed in with no streaks.
In a separate bowl, beat the eggs and milk until well combined, then stir in the melted butter. The mixture should look smooth and cohesive — no separated butter floating around.
Add the wet ingredients to the flour mixture and stir just until everything comes together. Don't overmix — a few lumps are fine and actually preferable to a tough cake.
Gently fold in the diced apples, making sure they're evenly distributed throughout the batter. Pour the whole mixture into your prepared pan and spread it level with a spatula.
For the topping, combine the 1/4 cup flour and brown sugar in a small bowl. Add the cold butter cubes and use your fingers or a pastry cutter to work everything together until it looks like coarse, chunky crumbs.
Scatter the crumb topping evenly across the surface of the batter. Don't worry about perfect coverage — rustic and uneven looks just right for this cake.
Bake until the top is golden brown and a toothpick inserted into the center comes out clean or with just a few moist crumbs clinging to it. The edges should pull away slightly from the pan sides.
⏱ 45 min
Let the cake rest in the pan for 10 minutes before cutting into wedges and serving. This brief cooling period helps the cake hold together better when sliced.
⏱ 10 min