Set oven to 350°F (175°C). Grease or line a 9-inch cake pan.
Whisk flour and cinnamon in a medium bowl; set aside.
Beat softened butter and sugar until light and fluffy, about 2–3 minutes.
Mix in eggs one at a time. Gradually add flour mixture until just combined.
Gently fold in diced apples with a spatula; batter will be thick.
Spread batter evenly into prepared cake pan.
Combine brown sugar, flour, and butter in a small bowl. Mix until crumbly.
Sprinkle crumble over batter. Bake 35–40 minutes, until golden and a toothpick comes out with moist crumbs.
Cool in pan for 20 minutes, then transfer to a rack. Serve warm or at room temp.