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Preheat oven to 325°F. Grease and flour a 10- to 12-cup Bundt cake pan.
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Whisk together flour, baking powder, baking soda, and salt in a medium bowl.
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Place sugar and lemon zest in a small bowl. Use your fingers to rub sugar and lemon zest together (releases lemon oil for more flavor).
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Place lavender and turbinado sugar in a spice grinder. Grind until it is a fine powder. Add powder to lemon-sugar mixture and stir or whisk to combine.
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Beat butter and sugar mixture 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 juice and beat until combined.
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Add flour mixture in 3 additions, alternately with buttermilk in 2 additions, beginning and ending with flour mixture.
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Pour batter into prepared pan and spread evenly in the pan.
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Bake for 55 to 60 minutes, or until a toothpick or cake tester inserted in the center comes out clean.
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Cool cake in pan for 12 minutes. Prepare glaze.
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Remove cake from pan onto a wire rack over parchment or waxed paper. Use a pastry brush to brush glaze on warm cake.
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Cool cake completely.