Make the lemon sugar syrup: In a small pot, bring sugar, water, and lemon zest to boil over medium heat. Boil until sugar is completely dissolved and syrup thickens slightly, about 3 minutes. Remove from the heat and stir in the lemon juice. Cool completely and remove any big pieces of lemon zest.
Place the berries in a bowl and toss with ¼ cup of the lemon syrup; set aside.
Make the lemon cream: Using a mixer, beat the heavy cream to stiff peaks, 2 to 3 minutes. Add the sour cream, ¼ cup lemon sugar syrup, and vanilla, and whisk back to stiff peaks, 1 to 2 minutes.
Make sponge cake (optional, can also use store bought cake): Preheat oven to 350°F. Prepare 8” round cake pan with butter and flour. In a mixing bowl, beat egg at high speed with electric mixer until thick and pale, 3 minutes. Gradually add sugar, beat until light and fluffy, 4 minutes.
In a small bowl, combine flour, baking powder and salt; gradually fold into batter.
In a small sauce pan, combine milk and butter; cook over low heat until butter melts. Gradually stir milk mixture and vanilla into batter.
Pour batter into prepared cake pan. Bake for 16 minutes or until toothpick comes clean. Cool on wire rack for 10 minutes. Remove from pan; cool completely on rack. Crumble into 1 inch pieces.
Assemble the trifle: In a bowl, combine the crumbled cake with ¼ cup each of the lemon syrup and the lemon cream to moisten. In four glasses, spoon a ¼ each of the cake, cream, and berries. Serve immediately or refrigerate overnight, without berries on top.