Preheat oven to 350°F (180°C). Spray an 8½x4½-inch loaf pan with baking spray with flour. Line pan with parchment paper, letting excess extend over all sides of pan.

In the bowl of a stand mixer fitted with the paddle attachment, beat butter and granulated sugar at medium speed until light and fluffy, 3 to 4 minutes, stopping to scrape bottom and sides of bowl. Add eggs, one at a time, beating until combined after each addition. Beat in sweet potato and 1 teaspoon (4 grams) vanilla.

In a medium bowl, whisk together flour, baking powder, and salt. With mixer on low speed, gradually add flour mixture to butter mixture, beating until just combined and stopping to scrape bottom and sides of bowl.

Spoon about 1 cup or 280 grams batter into each of 2 bowls. Stir melted chocolate into one bowl until combined. Stir cinnamon and pie spice into second bowl. Leave remaining batter as is.

Using a 1½-tablespoon spring-loaded scoop, scoop each batter in an alternating pattern into a single layer in prepared pan. Firmly tap pan on a kitchen towel-lined counter a few times to level batter. Continue scooping batters, tapping pan after each layer, until all batters are used. Using a butter knife or offset spatula, gently swirl batters together.

Bake until a wooden pick inserted in center comes out clean and an instant-read thermometer inserted in center registers at least 205°F (96°C), 55 minutes to 1 hour. Let cool in pan for 10 minutes. Using excess parchment as handles, remove from pan, and let cool completely on a wire rack.

In a small bowl, whisk together confectioners’ sugar, 1½ tablespoons (22.5 grams) milk, and remaining ½ teaspoon (2 grams) vanilla until smooth; whisk in remaining ½ teaspoon (7.5 grams) milk to reaches spreading consistency if needed. Spoon glaze onto cooled cake. Refrigerate in an airtight container for up to 3 days.