Preheat oven to 350ºF degrees. Coat an 8-inch loaf pan with butter then dust bottom and sides with flour; set aside.
Add flour and baking powder to a fine mesh sieve. Shake over a piece of parchment paper to sift the flour. Repeat this 1 to 2 more times.
Using an electric hand or stand mixer, beat the egg whites until foamy, then gradually spoon in the ½ cup (100 grams) sugar while continuing to beat on high until mixture becomes glossy and stiff peaks form.
In a separate bowl, add the butter and beat on medium-high until light and fluffy. Add remaining sugar and lemon zest then beat until fluffy. Add egg yolks and elderflower syrup then beat until well-blended and fluffy.
Gently spoon the sifted flour mixture into the bowl with butter-egg mixture. Gently stir to combine (batter will be very thick).
Add 1/3 of the whipped egg whites to the bowl and stir until the batter loosen up a bit.
Add remaining whipped egg whites, and gently fold in just until the batter is smooth and there are no visible traces of flour.
Spoon batter into prepared pan and bake on center oven rack until deep golden and a skewer inserted in the center comes out clean, 55 to 65 minutes. Remove from oven and let cake cool in pan 15 to 20 minutes, then remove from pan and continue cooling on a wire rack. Leftovers can be stored in a ziptop bag for 3 to 5 days.