MAKE THE LEMON CURD: In a small saucepan over medium heat, whisk together the egg yolks, granulated sugar, lemon juice, and lemon zest until combined and smooth.
COOK THE CURD: Stir constantly until the mixture thickens enough to coat the back of a spoon, about 5–7 minutes.
FINISH THE CURD: Remove from heat and stir in the cubed butter until fully melted and the curd is silky.
CHILL THE CURD: Pour the lemon curd into a shallow dish, cover, and refrigerate until firm, at least 2 hours or overnight.
FREEZE THE CURD: Once chilled, scoop the lemon curd into 4–6 small balls, about 1 tablespoon each, and freeze until solid.
PREPARE THE OVEN AND RAMEKINS: Preheat the oven to 425°F (220°C). Butter and lightly flour 4 ramekins, tapping out any excess flour.
MAKE THE BATTER: Melt the butter and white chocolate chips together in a double boiler or microwave in 30-second intervals, stirring until smooth.
WHISK THE EGGS: In a separate bowl, whisk together the eggs, egg yolks, granulated sugar, salt, and vanilla extract until slightly thickened and pale in color.
COMBINE WET INGREDIENTS: Slowly whisk the melted chocolate mixture into the egg mixture until fully incorporated.
ADD THE FLOUR: Sift the all-purpose flour into the batter and gently fold it in until just combined, taking care not to overmix.
ASSEMBLE THE CAKES: Spoon a heaping tablespoon of batter into each prepared ramekin, place a frozen lemon curd ball in the center, and cover with more batter until the ramekin is about three-quarters full.
BAKE: Bake the cakes for 12–14 minutes until the edges are set but the centers still jiggle slightly when the ramekins are gently shaken.
UNMOLD AND SERVE: Let the cakes cool for 1 minute, then carefully invert onto serving plates. Dust with powdered sugar and serve immediately.