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Preheat oven to 350F.
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Whisk the milk and vinegar together and set aside for about 5 minutes to sour.
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Beat butter and sugar together until fluffy.
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Add in eggs and vanilla bean paste one a time until incorporated. Stir in lemon zest and limoncello.
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Sift in flour, baking powder, and salt, alternating with the sour milk. Stir gently just until combined.
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Fill 18 cupcake liners with batter and bake at 350 for 14-17 minutes or until toothpick inserted in the center comes out clean.
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Allow to cool on a wire rack while you make the limoncello curd.
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In a double boiler, heat eggs and sugar, stirring constantly, until smooth.
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Add lemon juice, zest, and limoncello and whisk until thick and creamy.
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Remove from heat and add butter, one tablespoon at a time, until completely incorporated.
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Strain through mesh strainer and allow to cool.
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Spread on top of limoncello cupcakes or use as cupcake filling. Or spread for your morning biscuit. Whatever.