. Pre-heat oven to 350 degrees F. Grease and flour or line with paper cups or parchment paper, 12 cup muffin tin.
2. Make the topping with the first four ingredients: Combine brown sugar, flour, melted butter and cinnamon in a small bowl and mix until crumbly; Set aside.
Combine the brown sugar, oil, egg and vanilla in a large bowl. Mix and set aside.
4. Sift together the flour, soda, salt and cinnamon. Blend the flour into the mixture, alternately with the buttermilk. Add the diced apple. Mix until just combined. Spoon into prepared muffin tins. Sprinkle generously with topping.
5. Bake for 30 minutes, until golden brown and the top springs back when touched.
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