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Preheat oven to 350˚F. Line a 12-cup muffin pan with paper liners
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Make the crumb topping by combining the melted butter, sugar, flour, oatmeal and salt. Stir with a fork until crumbs form. Set aside.
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Combine flour, baking powder and salt in a medium size bowl. Set aside.
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If using frozen blueberries, place them in a small bowl and return to freezer until ready to use. If using fresh berries, place in a small bowl and set aside.
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In the bowl of an electric mixer, beat butter for 1 minute until soft and fluffy. Add sugar and continue to beat for 3 minutes. Add eggs, one at a time and beat just until incorporated. Add buttermilk, vanilla and lemon zest and mix on low until combined.
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Remove bowl from mixer. Add flour mixture and stir, by hand, until flour disappears. Do not over mix or muffins will be tough. Gently stir in blueberries.
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Divide mixture evenly in the prepared muffin pan. Sprinkle with the crumb topping.
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Bake at 350˚F for 22 to 25 minutes or until light golden and a toothpick inserted into the center comes out clean. Allow muffins to cool for 5 minutes in the pan before removing to a wire rack. Drizzle lightly with the lemon glaze.
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For lemon glaze, combine powdered sugar and lemon juice in a medium size bowl. Stir with a fork or whisk until smooth.