• Make your lilac syrup and have it ready.
• Preheat the oven to 350°F and line a cupcake pan with paper liners.

For the cupcakes:

• Make lilac sugar: Pulse ¾ cup fresh lilac flowers with ¾ cup sugar until fragrant and lightly broken down.
• Whisk together dry ingredients: In a bowl, whisk together 1 ½ cups flour, 1 tsp baking powder, ½ tsp baking soda, and ½ tsp salt. Set aside.
• Whip eggs to ribbon stage: Beat 2 eggs on medium speed using electric beaters or a stand mixer with the whisk attachment until thick, pale, and voluminous, about 5 minutes. With the mixer running, gradually add the lilac sugar you made in step 1, followed by 2 Tbsp lilac syrup.

  The mixture will be thick, frothy, and pale, and should fall in thick ribbons that briefly sit on the surface before dissolving
• Fold in dry + wet ingredients: With mixer running on low speed, alternate adding the flour mixture and the 1 cup buttermilk. Start with about ⅓ of the flour mix, then about ½ of the buttermilk, continuing until all the ingredients are gently folded in.

  Mix gently to avoid deflating all the air you've built into the batter, and stop mixing as soon as no streaks of flour remain. You might find it helps to finish mixing by hand, folding gently with a spatula. The batter will be somewhat thin and full of bubbles.
• Bake: Divide into lined cupcake tins and bake until the tops spring back lightly when touched, 13-17 minutes. Allow to cool completely.

For the buttercream:

• Clean and dry your mixer & whisk attachment.
• Heat syrup: In a small saucepan over medium-low heat, cook ¾ cup lilac syrup to 235–240°F ("soft ball" stage).
• Beat egg whites: While syrup is heating, add 3 egg whites to mixer with the whisk attachment and whip on medium speed until soft peaks form.
• Stream in the hot syrup: With the mixer running on medium speed, slowly and carefully pour the hot lilac syrup into the egg whites in a thin, steady stream. Aim for the side of the bowl to avoid splattering.
• Whip to a glossy meringue: Increase speed to medium-high and continue whipping for about 5 minutes, or until the mixture becomes thick, glossy, and has cooled to about 90° F (just slightly warm to the touch).
• Add the butter: With the mixer running (low to medium-low), add 8 oz soft butter, about a tablespoon at a time, allowing each addition to incorporate before adding more. The mixture may look curdled or soupy at first. Keep mixing, and it will transform into a smooth, silky buttercream.
• Decorate: Pipe or spread the frosting on your cupcakes and finish by topping with fresh flowers, candied lilacs, or sprinkles.