Do not skip the simmer: the binder needs to bubble gently for a full 60 to 90 seconds so it turns glossy and sticky enough to hold the oats together. Give the bars the full cooling time before cutting — this is what lets the structure set. Store in an airtight container in the fridge for up to 2 weeks or freeze individually wrapped in parchment for up to 3 months. For a vegan version, swap the honey for maple syrup and use dairy-free chocolate chips. If the mixture seems dry, stir in a tablespoon of warm water.