Preheat oven to 375 degrees.
Add flour, sugar, cinnamon and salt to a food processor and pulse several times.
Butter a 9 by 9-inch baking dish. Line with parchment paper and butter parchment paper, set aside.
In the bowl of an electric stand mixer fitted with the paddle attachment cream butter and granulated sugar until pale and fluffy. Mix in egg and vanilla.
Pour batter into prepared pan and spread into an even layer. Remove crumble from refrigerator and sprinkle evenly over top.
Bake in preheated oven until toothpick inserted into center comes out clean (center also shouldn't fall when lightly pressed), about 30 - 35 minutes.