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Preheat oven to 350 degrees F. Lightly grease a 10-inch springform pan.
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Whisk together flour, baking powder, and salt in a bowl.
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Beat butter and sugar in a large bowl until light and fluffy.
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Add eggs, one at a time, beating well after each addition.
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Add lemon extract and beat to combine.
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Add flour mixture and milk and beat on low speed until just combined. Batter will be thick.
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Spread batter evenly in prepared pan.
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Arrange apple slices and walnuts on top of batter.
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Bake for 30 to 35 minutes, until lightly browned and a toothpick or cake tester inserted into the middle comes out clean.
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In the last 10 to 15 minutes of baking, make the glaze.