Preheat oven to 375F; lightly grease an 8 by 8-inch baking dish with butter and line the bottom with parchment paper.
For the crust, whisk together the flour, brown sugar, cinnamon, and salt. Use a fork or pastry cutter to cut the butter into the flour mixture until there are some pieces the size of peas, but the mixture comes together as dough when pressed together in your palm.
Transfer ⅔ of the crust mixture to the prepared dish; spread it out in the bottom and then lightly press it down. Bake 10 minutes.
Meanwhile, stir together the apple, sugar, cinnamon, nutmeg, and salt in a medium bowl and let it sit until it forms a syrup in the bottom of the bowl, about 20 minutes. Stir in the eggs, cream, flour, and vanilla.
Pour the apple mixture onto the prepared crust; crumble the remaining ⅓ of the flour mixture on top.
Bake until the cake is golden and a wooden pick inserted inside comes out clean, about 30 to 40 minutes.
Cool on a wire rack before cutting.
Recipe by An Edible Mosaicâ„¢ at https://www.anediblemosaic.com/streusel-topped-apple-cinnamon-custard-cake/