Make the Champagne reduction: In a small saucepan, simmer 1 cup Champagne over medium heat until reduced to ¼ cup (about 10 minutes). Let cool.
Prep: Preheat oven to 350°F. Grease and flour two 9-inch round cake pans.
Dry ingredients: Whisk together flour, baking powder, and salt.
Cream: Beat butter and sugar until light and fluffy (3-4 minutes). Add eggs one at a time, beating well after each. Mix in vanilla and the cooled Champagne reduction.
Combine: Alternately add dry ingredients and milk to the butter mixture, beginning and ending with dry ingredients. (Option: Gently fold in 1 cup of berries.)
Bake: Pour into prepared pan and scatter remaining berries on top. Bake 45-50 minutes until a toothpick comes out clean.
Make syrup: While cake bakes, combine ½ cup Champagne, ¼ cup sugar, and lemon juice in a small saucepan. Simmer 5 minutes until slightly thickened.
Finish: While cake is still warm, poke holes with a skewer and brush with Champagne syrup. Let cool completely before frosting.
Tip: Chill the cake to minimize crumbs while frosting