Heat oven to 350°F (176ºC) with rack set in middle position. Coat an 8½ × 4½-inch loaf pan with nonstick spray. Line pan with parchment paper, leaving a 2-inch overhang on two sides; coat paper with nonstick spray.
Whisk together flour, cornstarch, cinnamon, baking soda, and salt in a medium bowl, set aside.
Whisk together buttermilk, peanut butter, honey, egg, and vanilla in a large bowl until smooth. Fold flour mixture into peanut butter mixture with a rubber spatula until combined.
Scrape batter into prepared pan and smooth top. Sprinkle peanuts and Demerara sugar evenly over top.
Bake until golden brown and a toothpick inserted in the center of the loaf comes out with few moist crumbs attached, 55–60 minutes. (You can also check doneness by temperature. The interior of the bread should register between 190ºF and 200ºF on an instant-read thermometer).
Let bread cool in pan on wire rack for 10 minutes, then turn bread out onto wire rack to cool completely, at least 1½ hours.