Line a small rectangular tin (about 18 × 12 cm, like a loaf tin) with baking paper, leaving some overhang to lift the bars out easily. Lightly grease the paper if you want clean, sharp edges when slicing.
In a large bowl, combine the rolled oats, protein powder, cocoa powder, and salt. Stir well so everything’s evenly mixed and free of clumps.
Add the peanut butter, honey, butter, coffee, and dark chocolate to a heatproof bowl. Set it over a pan of gently simmering water (a bain-marie) and stir until smooth, glossy, and chocolatey.
Pour the melted mix into the dry ingredients and stir until all the oats are coated and starting to clump. If it feels too dry, add milk a tablespoon at a time until it presses together easily.
Spoon the mixture into your prepared tin and press it down firmly with the back of a spoon or spatula. Pack it tightly so the bars hold together once chilled.
Scatter the chopped pecans over the top and press them in lightly, or spread another 30 g of melted dark chocolate over the surface first for a richer finish.
Refrigerate for at least 1 hour, or freeze for 20 minutes until firm and sliceable.
Lift the slab from the tin and cut into 8 bars. Store in an airtight container in the fridge for up to a week, or freeze for longer.