Sunny, springy weather calls for an all-yellow, all-lemon dessert.
Preheat the oven to 350°. Grease a 9-by-13-inch baking pan. Line the bottom with parchment paper and grease again.
For the cake, in a medium bowl, whisk both flours, the baking powder, salt, and baking soda. In a large bowl (or in the bowl of a stand mixer), using your fingertips, rub the sugar and lemon zest until blended. Add the butter and vanilla. Using an electric mixer or a stand mixer fitted with a paddle attachment, beat on medium to medium-high speed, stopping occasionally to scrape down the bowl, until light and fluffy, 3 to 5 minutes. Reduce the speed to low. Add the eggs and egg yolk, one at a time, beating until blended between each addition. In a small bowl, stir the lemon juice and sour cream.
Starting with the dry ingredients, alternately add the dry ingredients in 3 additions and the lemon sour cream in 2 additions. Stop mixing when only a few streaks of flour remain. Using a spatula, finish mixing the batter until blended. Transfer to the pan and smooth the top of the batter.
Bake the cake, rotating the pan halfway through baking, until a toothpick inserted in the center comes out with a moist crumb or two attached, 30 to 35 minutes. Let the cake cool in the pan on a wire rack for 10 to 15 minutes. Invert the cake onto the rack and peel off the parchment. Turn the cake right-side up.
Meanwhile, for the simple syrup, heat 1/4 cup water and the sugar over medium, stirring until the sugar dissolves, about 3 minutes. Off heat, stir in the lemon juice. Let stand until cool, about 20 minutes.
Using a toothpick, poke holes all over the top of the cake. Using a pastry brush, brush the cake with the simple syrup.
For the buttercream, in a large bowl, using an electric mixer or a stand mixer fitted with a paddle attachment, beat the butter and lemon zest on medium speed until fluffy and smooth, about 2 minutes. Beat in the salt. Alternately add the confectioners’ sugar, 1 cup at a time, and the lemon juice, 1 to 2 tbsp. at a time, beating until blended between each addition. Continue beating until light and fluffy, 3 to 5 minutes. Beat in the food coloring.
Spread the buttercream over the cooled cake. Just before serving, sprinkle with the sparkling sugar.