Position oven rack in center of oven. Preheat to 350 degrees farenheit. Generously butter and flour (with all-purpose flour) 2 8”-inch round cake pans (do not use cake flour for dusting).
Sift cake flour, baking powder and salt together into a medium bowl.
Mix ¼ cup sugar with lemon zest until well-combined in small bowl.
In the bowl of your stand mixer with the paddle attachment, beat butter, ¼ cup sugared lemon zest, and lemon extract on medium about 1 and a half minutes.
Add remaining 1 ½ cups sugar and beat until smooth, about 1 and a half minutes. Beat in ¼ of the milk until just blended.
On low speed, add flour mixture alternately with remaining milk in alternating batches, scraping down bowl between each addition with a rubber spatula; beat until just blended – do not overmix.
Either remove batter into separate bowl, clean out stand mixer bowl and move on. OR, use hand mixer and separate bowl for next step.
In another bowl, beat the egg whites with electric mixer on medium speed just until foamy. Add the cream of tarter, increase speed to medium-high and beat until the whites form stiff peaks when the beaters are lifted.
Add ¼ of the egg whites to cake batter and fold in with whisk or rubber spatula.
Continue to gently fold in the egg whites, ¼ at a time, being careful not to deflate the mixture by mixing too hard or too fast.
Divide batter evenly between prepared pans, smoothing the tops with a spatula. Place layers in pre-heated oven and bake for 28-30 minutes or until a toothpick inserted in the cake comes out clean.
Let the cake layers cool in pans on a cooling rack for at least 10 minutes until cooled. Run a knife around the inside of the pans and carefully invert onto the racks. Flip right side up and let cool completely.
For the Lemon Curd Filling:
Crack whole egg and yolks into a medium-small size mixing bowl. Set aside.
In a saucepan, combine sugar, flour, cornstarch, and salt. Add water, lemon extract, and lemon zest. Mix.
Cook over medium-low heat, stirring constantly, until mixture boils.
Beat 1/2 cup of the mixture into the bowl with the whole egg and yolks; return to saucepan.
Simmer 3 minutes, then stir in butter until melted.
Pour or spoon into tart let shells; cool one hour.
For the Lemon Frosting:
In a medium bowl, beat the butter and lemon zest with an electric mixer on medium speed until light and fluffy.
Add the confectioners’ sugar in batches, scraping down sides getting all ingredients from the bottom a couple of times, and beat until light and fluffy. Add the lemon extract and beat for 1 minute.
To Assemble
With the palm of one hand pressed gently on the top of one of the cake layers, cut in half horizontally with a long, serrated knife. Repeat with other cake layer.
Put one of the four cake layers on a serving plate, cut-side up. With an off-set spatula, or table knife, spread a generous 1/3 cup chilled lemon curd on top of the cake layer.
Lay another cake layer on top. Spread with another generous 1/3 cup chilled lemon curd on top. Repeat with 3rd cake layer, using the last 1/3 cup chilled lemon curd. Top with 4th layer of cake.
Frost the cake with 1/3 of prepared frosting to evenly coat sides and top. This is a “crumb coat” to make the rest of the frosting apply easier. Chill for about an hour.
Spread remaining frosting decoratively over sides and top of cake. Scatter lemon zest on top to garnish, if desired.
