« » Make the cake: Preheat oven to 325 degrees, with rack in lower third of oven. Sift flour and 1/2 cup sugar into a bowl.
Whisk whites with a mixer on medium speed until frothy, about 1 minute. Add lemon zest and juice, cream of tartar, vanilla, and salt; continue whisking until soft peaks form, about 2 1/2 minutes. With mixer running, gradually add remaining cup sugar.
Increase speed to medium-high; continue whisking until firm, not stiff, peaks form, about 2 minutes. Sprinkle whites with 1/3 of the flour-sugar mixture. Using a rubber spatula, gently fold to combine. Sprinkle remaining flour-sugar mixture over whites in 2 additions; gently fold to combine.
Transfer batter to a 10-inch angel food cake pan with legs. Gently run a knife through the center of the batter to remove any air bubbles. Bake until a tester inserted into center comes out clean, 45 to 50 minutes.
Remove pan from oven, and invert onto its cooling legs (if your tube pan doesn't have legs, invert it over the neck of a wine, or similarly shaped, bottle to cool); let cool, 1 1/2 to 2 hours. Run a knife around the inner and outer edges of cake to remove. Invert onto a serving platter. (Use a knife to separate cake from bottom of pan.) Let cool on a wire rack. Unfrosted cake can be stored in an airtight container for up to 2 days.
Make the lemon cream: Prepare an ice-water bath. Whisk lemon juice, sugar, flour, and salt in a small saucepan over medium-high heat. Bring to a boil; whisk constantly for 1 minute, until it thickens. Transfer to a heatproof bowl set in ice-water bath to cool completely, stirring occasionally. Meanwhile, whisk cream and lemon zest with a mixer on medium speed until medium peaks form, about 3 minutes. Gently fold whipped cream into juice mixture in thirds. Refrigerate lemon cream, up to overnight.
Make the candied lemon-peel flowers: Combine sugar and water in a large saucepan over medium-high heat, stirring until sugar dissolves. Bring to a simmer. Meanwhile, using a vegetable peeler, peel 3 lemons to create 6 long strips (at least 5 inches long and about 1/2 inch wide). Remove pith from strips using a paring knife. Cut tops and bottoms off remaining 3 lemons; cut each in half lengthwise. Remove flesh. Cut out 6 flowers from the peels using a 2-inch flower-shaped cookie cutter and remove pith; cut indentations into petals for definition. Punch a hole in the center of each flower using a 1/4-inch plain round piping tip.
Add lemon-peel strips and flowers to the syrup. Gently simmer until translucent and tender, about 35 minutes. Remove peels from syrup using a slotted spoon, and transfer to a wire rack set over a parchment. Let dry slightly, about 30 minutes.
Trim strips to 5-inch lengths, and roll each to form centers of flowers; fit into flower holes and let stand on wire rack until slightly dry but still sticky, about 30 minutes.
Frost cake with lemon cream. Garnish with candied flowers. Serve.