Combine the flour, brown sugar, cinnamon, and salt in a medium bowl.
Add the butter, and mix with a pastry blender, a fork, or your fingers until the mixture forms crumbs. Set aside, or refrigerate if your kitchen is warm.
To make the cake:
Preheat oven to 350°F (175°C). Grease a 13"x18" half-sheet pan.
Whisk together the flour, baking powder, cinnamon, and salt. Set aside.
Using an electric mixer on medium speed, beat the butter, brown sugar, and sugar until light and fluffy. Add the eggs, two at a time, mixing well after each addition. Mix in the vanilla.
Reduce mixer speed to low. Add the flour mixture in three portions, alternating with two portions of milk. Mix just until combined.
Stir in the apples and pecans.
Transfer the batter to the prepared pan, and spread evenly. Sprinkle the crumb topping over the top of the batter.
Bake for 25 to 30 minutes, or until the cake is golden brown and a pick inserted into the center comes out clean.
Cool in the pan on a wire rack.
To make the glaze:
Combine the confectioners' sugar and 3 tablespoons of milk until thoroughly mixed. Add more milk as needed for the desired consistency.
Drizzle the glaze over the cake before serving.
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Notes
Peeling the apples is optional, so just go with your preference. Use firm, baking apples like Honeycrisp, Granny Smith, or Gala.
Pecans can be substituted with walnuts or omitted for a nut-free version.
For extra flavor, use apple cider in place of milk in the glaze.
Storage: Keep covered at room temperature for up to 2 days, refrigerate up to 5 days, or freeze up to 2 months.