Wrap 1 large sweet potato in foil and roast at 425 degrees F for 45 to 50 minutes, until it collapses when pressed. Scoop out the flesh and mash it smooth to get 1 cup.

Let the mash cool to room temperature. Heat the oven to 350 degrees F and line a 9x9 inch pan with parchment, leaving an overhang on two sides.

In a large bowl, whisk the mashed sweet potato, eggs, maple syrup, peanut butter, melted butter and vanilla until completely smooth with no streaks of peanut butter.

Add the oats, flour, baking powder, cinnamon and salt. Stir until every oat is coated. The mixture will be thick and stiff, more dough than batter.

Fold in the chocolate chips or raisins, if using.

Scrape the mixture into the lined pan and press it down firmly and evenly with the back of a spatula or a damp palm, working right into the corners.

Bake 28 to 32 minutes, until the top is golden and the center feels set when pressed lightly. The edges will darken slightly and pull away from the parchment.

Cool in the pan for at least 1 hour, until completely at room temperature.

Lift the slab out by the parchment overhang and cut into 12 bars on a board, pressing the knife straight down for each cut.