Preheat oven to 350℉. Prepare baking pan with nonstick cooking spray or paper liners in muffin tin sprayed with nonstick cooking spray.
To make the Streusel: In the bowl of a standing mixer fitted with the paddle, combine the flour with brown sugar and salt. Add the butter pieces and mix at medium-low speed until the mixture resembled 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.
To make the cake: In the mixer bowl, combine the flour with 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 apples and beat just until incorporated.
Pour into baking pan or scoop in muffin tins. Press the streusel into clumps and sprinkle on top. Bake in pan for about 45 minutes and muffins for about 30 minutes, or until golden and a toothpick comes out clean. Cool slightly before serving.
