Allow eggs to stand at room temperature for 30 minutes. Meanwhile, grease the bottom of a 15x10x1-inch baking pan; line with waxed paper. Grease and flour the waxed paper and sides of pan; set aside. In a small bowl, stir together flour and baking powder; set aside.
Preheat oven to 350 degrees F. In a large bowl, beat eggs with an electric mixer on high speed about 4 minutes or until thick. Gradually add granulated sugar, beating on medium speed for 4 to 5 minutes or until light and fluffy. Add the flour mixture; beat on low to medium speed just until combined.
In a small saucepan, combine milk and butter; heat and stir until butter melts. Add milk mixture to egg-flour mixture; beat until combined. Pour batter into the prepared pan.
Bake for 20 to 25 minutes or until cake springs back when lightly touched. Immediately loosen edges of cake from pan; turn out onto a clean kitchen towel sprinkled with powdered sugar. Remove waxed paper. Cool completely. Cut cake crosswise into thirds; set aside.
Place one of the cake layers on a serving plate. Spread with lemon curd. Top with another cake layer; spread with raspberry preserves. Top with remaining cake layer. Frost top and side of cake with the remaining sour cream frosting. Pipe a border around edges of cake. Cover and chill for 2 to 4 hours before serving.
If desired, for garnish:
Place thyme sprigs in a pie plate; dust with powdered sugar. Place raspberries on a plate; dust with powdered sugar. Garnish cake with sugared thyme sprigs and raspberries.
Sour Cream Frosting
Allow butter to stand at room temperature for 30 minutes. In a large mixing bowl beat butter with an electric mixer on medium to high speed for 30 seconds. Beat in sour cream and vanilla. Gradually add powdered sugar, beating well. Beat in enough of the milk to make frosting piping consistency.