Line a bread tin or other baking dish with parchment paper and set aside.
Add the chocolate, peanut butter, maple syrup, coconut oil, and vanilla to a saucepan. Melt over low heat, stirring occasionally, until the chocolate is just melted. The mixture will look seized when mixed.
Remove from the heat and mix in the puffed millet. If your pot is big enough you can mix in the pot, and otherwise transfer to a mixing bowl.
30 grams (1 cup) puffed millet
Press the mixture firmly into the lined bread tin. Refrigerate for at least two hours to firm up fully before slicing and serving.
Notes
Peanut butter: I haven't tried making these with ultra-processed peanut butter. Natural peanut butter will have just peanuts and maybe salt in the ingredient list, with no added sugar or oils.
Seized chocolate: it will look like the chocolate has seized, but that's because of the peanut butter and maple syrup mixing (it's actually the peanut butter seizing). This is normal and a big part of what makes for a softer texture when the bars are frozen.
Storage: keep in a sealed container in the refrigerator for up to one week. The bars should be kept chilled and served cold.
Freezing: place the bars in an airtight container and freeze up to two months. They can be served frozen or thawed in the refrigerator.