Beat softened butter and sugar for 5–6 minutes until pale and fluffy.
Mix in 5 room-temperature eggs one at a time.
Add lemon extract, vanilla, and optional zest.
Alternate adding flour and 7Up, mixing gently.
Butter and flour a bundt pan thoroughly.
Bake at 325°F for 1 hour 10 minutes or until toothpick is clean.
Let rest briefly, then turn onto a rack.
Mix milk, powdered sugar, and lemon flavor; drizzle over cooled cake.
