Lightly butter the bottom and sides of an 8-by-8-inch baking pan. Line with parchment paper (the butter will help the parchment not move around).

In a mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, beat butter, sugar, peanut butter, honey and vanilla until mixture is fluffy and a shade or two lighter in color.

Scrape down the sides of the bowl, then add egg and egg yolk, mixing until fully incorporated.

Add flour, salt, and baking soda and mix on low speed until almost incorporated, then add oats and mix briefly just to uniformly distribute them. Add m&ms and chocolate chips and stir in with a rubber spatula, scraping any bits of dry flour that might be left on the bottom or sides of the bowl.

Dump dough into prepared baking pan, using spatula to press it evenly all the way to the edges. Cover and chill for at least 2 hours or overnight.

Preheat oven to 350 degrees F. Let chilled dough sit at room temperature for a bit while the oven preheats.

Bake for 33 to 37 minutes or until top is puffed and no longer raw looking, and edges are just starting to brown.

Remove from oven and place pan on a wire rack to cool, then use parchment to lift the bars out of the pan to place on the rack to cool completely before slicing into squares Leftover bars will keep, refrigerated in an airtight container, for up to 5 days.