How to Make It
Preheat the oven to 350°. In the bowl of a standing mixer fitted with the paddle, combine the flour with the brown sugar and salt. Add the butter pieces and mix at medium-low speed until the mixture resembles coarse meal; continue mixing the streusel until very small clumps form. Transfer the streusel to a large plate and refrigerate until it is well chilled, about 10 minutes.
Line 18 standard-size muffin cups with paper liners. Spray the liners with vegetable oil cooking spray. In the mixer bowl, combine the flour with the granulated sugar, salt, baking powder, baking soda and cinnamon. Add the butter pieces and beat at low speed until the mixture resembles coarse meal. Add the sour cream and beaten egg and beat until the batter is smooth. Add the diced apple and beat just until incorporated.
Fill the muffin cups halfway with the crumb cake batter. Press the streusel into clumps and sprinkle on top. Bake the crumb cakes in the center of the oven for about 30 minutes, until risen, golden and springy to the touch; rotate the pans halfway through baking. Let the crumb cakes cool slightly before serving.