1. Line a large baking sheet with parchment. Combine butter, sugar and corn syrup in a small saucepan over medium heat and cook, stirring occasionally, until butter has melted. Bring to a boil, add condensed milk and continue to boil for 5 minutes, stirring to prevent scorching. Remove from heat and stir in vanilla.
2. Place popcorn in a large bowl. Pour butter mixture over popcorn and stir gently to coat completely. Set aside until mixture has cooled enough so that it's still sticky but won't melt candy, 3 to 5 minutes. Toss in peanuts, toffee bits and white chocolate chips.
3. When popcorn mixture is cool enough to handle, grease hands with cooking spray or vegetable oil and roll mixture into 2-inch balls (use about 1/4 cup per ball). Place balls on baking sheet. Sprinkle with salt, if desired. Let cool completely. Store in an airtight container at room temperature.