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1
Preheat oven to 350F (180C).
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2
Prepare the topping: In a medium bowl, combine brown sugar, flour, butter and cinnamon.
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3
Use a pastry blender or fork to mix until mixture is coarse and crumbly and the butter is well incorporated.
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4
Stir in the nuts (if using) and mix well.
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5
Set aside.
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6
Prepare the cake: In another bowl, combine flour, baking powder and baking soda.
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7
In a large mixer bowl, cream butter and sugar on medium speed until smooth and fluffy.
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8
Beat in eggs, one at a time, beating well after each addition.
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9
Add vanilla and yogurt and beat on low speed until blended.
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10
Beat in flour mixture, beating on low speed until well blended.
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11
Spoon half the batter into the prepared baking pan.
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12
Sprinkle evenly with 1/4 cup (50 mL) of the crumb topping, then the apples, and then 1/2 cup (125 mL) of the crumb topping.
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13
Spoon on the remaining batter and top with the remaining crumb topping.
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14
Bake in preheated oven for 55 to 60 minutes or until a toothpick inserted in the center comes out clean and dry.
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15
Let cool in pan on a wire rack for 10 minutes.
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16
Put a large plate over top and carefully invert.
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17
Remove pan.
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18
Place another plate on top of cake and invert so that crumb topping is facing up.
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19
Let cool completely.