Make the Lemon Curd: Pour enough water into a saucepan to reach a depth of 2 inches and bring to a boil.
In a mixer fitted with a whisk attachment (or using a hand mixer), whip the eggs and sugar together until very light yellow and fluffy. Mix in the lemon juice and lemon zest.
Rest the mixing bowl on the saucepan, with the bowl's base resting above not in the simmering water (pour out some water, if necessary). Cook, whisking occasionally, until the mixture is thickened and custard-y, about 10 minutes.
Meanwhile, half fill a large bowl with ice and cover with cold water. Remove the bowl of lemon curd from the hot water and whisk in the butter until melted. Rest the bottom of the bowl in the ice bath and let cool, folding the mixture occasionally to cool and thicken it.
In a mixer fitted with a whisk attachment (or using a hand mixer), whip the cream until stiff. Fold it into the cooled lemon custard. Cover and refrigerate until ready to serve. (The recipe can be made to this point and kept refrigerated up to 3 days in advance.)
Make the Cake: Preheat the oven to 350 degrees F.
Butter the jelly roll pan and line it with parchment paper. Butter the paper to assure release. Have some water simmering in a saucepan. Place the eggs, yolk, sugar, and vanilla in the bowl of a standing mixer (don't attach it to the mixer yet) and whisk briefly. Set the bowl over the simmering water and stir the mixture until warm and sugar looks dissolved. Attach the bowl to the mixer and whip until light and fluffy, about 6 minutes.
Meanwhile, sift together the flour and baking powder 3 times. Sift it over the whipped egg mixture while folding it in; then fold in the coconut. Drizzle in the clarified butter, while folding the batter.
Immediately pour the batter into the prepared pan and smooth the top with a spatula. Bake until lightly browned and starting to pull away from the edges of the pan, about 14 to 16 minutes. Let the cake rest in the pan 1 minute. Run a knife along the edge to release the cake; then flip it out onto a tea towel or parchment paper. Brush the paper lining with water and let soak 2 minutes before peeling it off the cake. Trim dry edges from the cake. Roll the cake up tightly with the towel, rolling away from you; then cool seam-side down on a rack.
Carefully unroll the cake and spread it with a thin layer of jam, followed by the lemon curd filling, leaving 1/2 inch empty at the far edge. Tightly re-roll the cake and place on a platter seam-side down. Dust with confectioners' sugar.
Whip the remaining 1/2 cup cream with the sugar until very stiff. Keep chilled. Using a pastry bag fitted with a star tip, pipe the whipped cream into rosettes down the center of the roulade. Place a candied violet on each rosette. Serve in 1-inch thick slices.