Line your pan with parchment paper, leaving overhang for easy lifting. In a microwave-safe bowl, melt the butter and peanut butter together in short bursts, stirring until smooth and fragrant, about 1-1.5 minutes. The mixture should be warm, nutty, and shiny.
Pour the melted mixture over the oats in a large bowl. Use a spatula or your hands to stir and coat the oats thoroughly until they look shiny and sticky, forming a dense, cohesive mixture.
Add honey and vanilla extract to the mixture, then stir well until everything is evenly combined. The mixture should be sticky enough to hold together when pressed.
Transfer the mixture into your prepared pan. Using a spatula or your hands, press firmly and evenly into the pan, creating a compact, smooth layer. This helps the bars hold their shape and look tidy.
If desired, melt the dark chocolate in short bursts in the microwave, then drizzle or spread it over the pressed mixture for a glossy, decadent topping. Sprinkle with sea salt flakes for added flavor and visual appeal.
Refrigerate the bars for at least 15 minutes, or until firm. This helps them set and makes slicing easier.
Once set, lift the bars out of the pan using the parchment overhang. Slice into 12 even squares with a sharp knife, wiping the blade clean between cuts for neat edges.
Serve immediately or store in an airtight container in the fridge. These chewy, nutty bars are perfect for a quick snack or a nostalgic treat.