Line an 8 x 8 pan with parchment paper leaving enough overhang to lift from pan. Set aside.
In a microwave safe bowl, melt together chocolate, condensed milk and butter using the melt function or on 50% power. Melt for 90 seconds, then stir. Repeat in 15 second increments until smooth stopping to stir each time.
Add the vanilla, marshmallows and pretzels. Mix until ingredients are evenly distributed.
Spread into pan using an offset spatula or knife. Immediately press 25 additional pretzels on top.
Chill for 4 hours. Lift from pan using parchment and use a sharp knife to cut into 25 even pieces.