Cook the sweet potato. Preheat the oven to 400°F. Pierce the sweet potato a few times with a fork and bake for 45–60 minutes, until very tender. You can also microwave it for 8–10 minutes, turning halfway. Let it cool, then scoop out the flesh and mash until smooth. Measure 1 cup.

Prep the pan. Lower the oven to 350°F. Line a 9×5-inch loaf pan with parchment paper, leaving some overhang to lift the bread out later.

Mix the dry ingredients. In a medium bowl, whisk together the flour, baking soda, baking powder, salt, cinnamon, and nutmeg.

Mix the wet ingredients. In a large bowl, whisk the melted butter and sugar until combined. Whisk in the eggs one at a time, then the mashed sweet potato, milk, and vanilla until smooth.

Combine. Add the dry ingredients to the wet and stir until just combined. Don't overmix. Fold in the chocolate chips and all but 2 tablespoons of the walnuts.

Bake the bread. Pour the batter into the pan and smooth the top. Bake for 50–60 minutes, until a toothpick in the center comes out clean or with a few moist crumbs. If the top browns too fast, tent loosely with foil.

Make the meringue. While the bread bakes, beat the egg whites and pinch of salt with a mixer until foamy. Gradually add the powdered sugar and beat until stiff, glossy peaks form.

Top and brown. Let the bread cool in the pan for 10 minutes and raise the oven to 400°F. Spread the meringue over the top of the bread and sprinkle with the reserved walnuts. Bake for 5–7 minutes, until the meringue peaks are golden brown.

Cool. Let the bread cool in the pan for 10 minutes, then lift it out with the parchment and cool completely on a wire rack before slicing.