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1
Make the Streusel Preheat the oven to 350.
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2
In the bowl of a standing mixer fitted with the paddle, combine the flour with the brown sugar and salt.
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3
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.
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4
Transfer the streusel to a large plate and refrigerate until it is well chilled, about 10 minutes.
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5
Make the Crumb Cakes Line 18 standard-size muffin cups with paper liners.
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6
Spray the liners with vegetable oil cooking spray.
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7
In the mixer bowl, combine the flour with the granulated sugar, salt, baking powder, baking soda and cinnamon.
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8
Add the butter pieces and beat at low speed until the mixture resembles coarse meal.
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9
Add the sour cream and beaten egg and beat until the batter is smooth.
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10
Add the diced apple and beat just until incorporated.
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11
Make the Crumb Cakes Fill the muffin cups halfway with the crumb cake batter.
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12
Press the streusel into clumps and sprinkle on top.
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13
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.
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14
Let the crumb cakes cool slightly before serving.
