Line two baking sheets with parchment paper and set aside.
In a medium saucepan, combine butter, milk, sugar, and cocoa powder. Bring to a rolling boil and let boil for 1–1.5 minutes, stirring constantly.
Remove from heat and quickly stir in peanut butter, vanilla extract, and salt until smooth.
Fold in the oats until fully combined.
Drop spoonfuls of the mixture onto the prepared baking sheets. Flatten slightly if desired. Let them set for 30 minutes at room temperature or refrigerate for quicker setting.
Melt the chocolate wafers in the microwave in 20-second intervals, stirring between each until smooth.
Dip the bottom of each cookie in melted chocolate or drizzle chocolate over the top. Place back on the parchment to set.
Sprinkle with sea salt flakes if using. Let the chocolate harden before serving.