Spray muffin pan with non-stick oil. Set oven to 350 degrees with rack in middle. Prepare brownie batter according to package directions. Add semi-sweet chocolate and combine. Divide batter among cups. Bake 10 min, rotate pan, bake 8-10 min more until center rises and is barely set. Cool completely.
Place 1/4 c white chocolate in medium microwaveable bowl. Heat 30 sec; stir to melt. Repeat in 10 sec increments, until completely smooth, being careful not to scorch. Add 1/4 c of peanut butter to melted chocolate; combine with a rubber spatula. Add cornflakes, lightly pressing to crush. Freeze 10-15 min until firm. Divide into 12 mounds (approx. a rounded tablespoon each), and quickly form into balls. If too soft to handle, return to freezer until firm.
In mixer bowl, beat butter 2-3 min. Microwave remaining 1/2 c white chocolate, as described above. Combine 1 c peanut butter with white chocolate and mix well. Scrape butter from the sides of mixer bowl and add peanut butter/chocolate mixture. Beat 1 min to combine, then scrape bowl. Add 1/2 c powdered sugar, vanilla and salt; combine. Scrape bowl, add remaining sugar and beat 2 min on high speed until light and fluffy. Add cream slowly, beating until smooth and shiny.
On work surface, press back of a measuring spoon into center of each brownie to make an indent. Place a crunch ball into it; top with a heaping spoon of frosting and run tip of spoon around for desired look. Garnish with chopped peanuts and serve.