Melt semisweet chocolate, peanut butter, and butter together in increments (about 30 seconds in the microwave, stirring between each) until smooth and fully combined. Then stir in vanilla extract.
Place Chex cereal in a large bowl. Pour the melted chocolate-peanut butter mixture over it, then gently stir until each piece is evenly coated.
Transfer the coated cereal into a large bag or container, add 1½ cups powdered sugar, and shake until coated. Spread on a sheet pan, chill in the refrigerator for about 10 minutes until it firms.
While cereal chills, melt the white wafers. Lay the holiday pretzels on parchment and drizzle the melted white chocolate over them. Immediately sprinkle colored sprinkles before it sets.
Once the cereal mix is chilled, transfer back to a bowl or bag, add the remaining ¾ cup powdered sugar and shake until fully white (snowy). Then spread out, sprinkle on M&M’s, peanut butter cups, and top with decorated pretzels. Serve or package.