In bowl of electric mixer, cream butter and brown sugar. Add eggs and vanilla; mix well. In separate bowl, whisk flour, baking powder, and salt. Add to butter mixture and mix until just combined. Fold in chocolate chips. Spread 2/3 of the mixture over bottom of 9×13-inch pan.
For cheesecake filling:
In separate bowl, whisk mascarpone, eggs, and powdered sugar. Spread over blondie dough in pan. Top by spoonful with remaining blondie dough. (Dough will not fully cover mascarpone layer.)
Press pretzels into dough. Bake 35 minutes, or until center is set. Immediately press M&Ms into top of bars. Cool completely. In microwave-safe bowl, heat white chocolate 30 seconds to melt. Continue to heat in 10-second increments if necessary. Using a piping bag or the back of a spoon, drizzle white chocolate over cooled bars. Immediately top with candy cane pieces. Allow white chocolate to set before cutting into bars and serving.
Photos and recipes courtesy of the Wisconsin Milk Marketing Board.