Poke a medium sweet potato all over with a fork and roast at 400F for 45 to 55 minutes, until a knife slides in with no resistance. For a shortcut, wrap it in a damp paper towel and microwave on high 5 minutes, flip, and go 4 to 6 minutes more.

Scoop out the flesh, mash it smooth, and measure 1 cup. If it was microwaved, let it drain in a fine mesh strainer for 10 minutes. Cool completely before using.

Heat the oven to 350F. Line a 9x9-inch pan with parchment paper, leaving overhang on two sides, and coat lightly with cooking spray.

In a medium bowl, whisk the flour, baking powder, baking soda, salt, cinnamon, nutmeg, and ginger for about 30 seconds.

In a large bowl, stir the melted butter and brown sugar until the mixture looks like wet sand. Stir in the egg and vanilla until smooth, then stir in the cooled sweet potato puree.

Fold the dry ingredients into the wet with a spatula, stopping as soon as the last streak of flour disappears. The batter will be thick. Fold in the chocolate chips if using.

Spread the batter into the pan and smooth the top. Bake 28 to 32 minutes, until the edges pull slightly from the parchment and a toothpick in the center comes out with a few moist crumbs.

Cool in the pan on a wire rack for 15 minutes, then lift out by the parchment and cool completely.

Whisk the powdered sugar and maple syrup until smooth and thick, adding milk ½ teaspoon at a time only if it will not spread.

Spread the glaze thin over the fully cooled slab and let it set for 20 minutes before cutting into 12 squares.