Line an 8-inch square pan with parchment, leaving overhang. Lightly mist with nonstick spray.
Warm condensed milk and 2.5 tbsp butter in a medium pot over medium-low, stirring until melted.
Whisk in the dry vanilla pudding mix until smooth and dissolved, about 1 minute.
Remove from heat. Stir in white chocolate chips and marshmallow creme until silky and fully melted.
Blend in vanilla extract and salt. Spread the custard fudge into the prepared pan and smooth the top.
Chill 20–30 minutes until the top feels set but still slightly soft to the touch.
Heat cream until steaming. Pour over chocolate chips with 1 tbsp butter and corn syrup; rest 2 minutes.
Stir ganache until glossy and smooth. Cool 5 minutes, then pour over the vanilla layer and spread gently.
Refrigerate until firm, 2–3 hours. Lift out with parchment and cut into 36 squares using a warm knife.