Prep: Heat oven to 335°F. Grease and line three 8-inch pans.
Curdle: Stir milk + vinegar; rest 5 minutes.
Wet: Whisk banana, yogurt, oil, sugars, vanilla. Stir in milk.
Dry: Whisk remaining dry ingredients.
Combine: Fold dry into wet just until smooth.
Bake: Divide evenly. Bake 28–32 minutes until set.
Cool: Rest 15 minutes, then wrap warm and chill.
Melt butter.
Stir in sugar + cinnamon; cook until glossy.
Whisk in cream; simmer 1–2 minutes.
Remove from heat; add salt + vanilla. Cool.
Once cold, whisk in milk as needed until spreadable.
Beat butter + cream cheese smooth. Add sugar gradually. Mix in vanilla + salt. Add milk as needed. Whip until fluffy and pipeable.
Pipe thin dam on each layer.
Spread thin frosting inside dam.
Spoon ⅓ goo over each layer.
Stack and chill briefly.
Frost exterior.
Optional: Drizzle or swirl goo on top.
