To make cake, heat oven to 350° F. Butter two 8-inch round cake pans; dust with flour, tapping out excess. In electric mixer with paddle attachment or with hand mixer, beat 8 tablespoons Kerrygold Butter and the sugar on medium speed until light. On low speed, beat in egg yolks, one at a time. Beat in milk and lemon extract; mixture may look curdled but will smooth out when flour is added. Whisk together flour, baking powder and salt. Add to butter mixture; beat on low speed just until smooth. In electric mixer with whisk attachment or with hand mixer, beat egg whites until stiff but not dry. Stir one-fourth of the egg whites into the batter; fold in remaining egg whites until no streaks remain. Spread batter in prepared pans, dividing it equally. Bake about 25 minutes or until a toothpick inserted into the cakes comes out clean. Cool in pans 5 minutes; turn out onto a rack and cool completely.
To make buttercream, lightly oil a heatproof glass measuring cup. In electric mixer with whisk attachment or with hand mixer, beat egg yolks on high speed until light. While yolks are beating, in small saucepan, heat sugar and corn syrup over medium heat, stirring constantly, until mixture comes to a full boil. Immediately pour into measuring cup. With mixer off, pour one-third of the sugar syrup onto egg yolks; beat on high speed for 5 seconds. Repeat with remaining syrup in 2 additions. Beat until cool. Add Kerrygold Butter, a few tablespoons at a time. Add lemon juice, zest and extract, beating until smooth. If not using immediately, refrigerate buttercream in airtight container; bring to room temperature and beat until smooth before using.
To Assemble Cake:
Trim 1 cake layer so that top of layer is flat; brush off any crumbs. With metal spatula, spread a layer of butter cream on trimmed layer. Place second cake layer, rounded side up, on top of first layer. Spread buttercream on top and sides of cake.