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Whisk together flour, sugar, and salt. Whisk together milk, eggs, and vanilla. Gradually pour milk mixture into flour mixture, whisking until smooth. Whisk in butter. Pour through a fine sieve into an airtight container; discard lumps. Refrigerate at least 2 hours or overnight.
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Lightly coat a 6-inch crepe pan or nonstick skillet with butter. Heat over medium heat until just starting to smoke. Remove from heat; pour about 2 tablespoons batter into center. Swirl to cover bottom. Reduce heat to medium-low; return pan to heat. Cook until edges are golden and center is dry, about 30 seconds per side.
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Slide crepe onto an overturned plate. Repeat with remaining batter, coating pan with butter as needed, and stacking crepes. Let cool.
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Place 1 crepe on a flat serving dish. Spread about 1/4 cup lemon curd mousse onto crepe. Top with 1 crepe. Continue layering crepes and mousse. (Use 15 crepes, ending with a crepe on top.) Refrigerate until firm, about 1 hour.
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Top crepe cake with whipped cream and 3 or 4 candied lemon slices.