Step 1: Preheat your oven to 325°F (160°C). Grease and flour a 10-12 cup Bundt pan thoroughly, ensuring all crevices are coated. Tap out any excess flour.
Step 2: In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, salt, cinnamon, and nutmeg. Set aside.
Step 3: In a large bowl, using an electric mixer, cream together the softened butter and granulated sugar until light and fluffy, about 3-5 minutes. Scrape down the sides of the bowl as needed.
Step 4: Add the eggs one at a time, beating well after each addition until fully incorporated. Mix in the mashed cooked sweet potatoes and 1 teaspoon of vanilla extract until well combined.
Step 5: Gradually add the dry ingredients to the wet ingredients in three additions, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix on low speed until just combined after each addition, being careful not to overmix.
Step 6: Pour the batter evenly into the prepared Bundt pan. Gently tap the pan on the counter a couple of times to release any air bubbles and even out the batter.
Step 7: Bake for 70-80 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean. The cake should be golden brown and spring back when lightly touched.
Step 8: Remove the cake from the oven and let it cool in the pan on a wire rack for 15-20 minutes. This allows the cake to firm up before inverting.
Step 9: Carefully invert the cake onto the wire rack to cool completely. Do not attempt to glaze the cake until it is fully cooled.
Step 10: While the cake cools, prepare the glaze: In a small bowl, whisk together the powdered sugar, milk, and ½ teaspoon vanilla extract until smooth. If the glaze is too thick, add more milk a teaspoon at a time; if too thin, add more powdered sugar.
Step 11: Once the cake is completely cool, drizzle the glaze evenly over the top, allowing it to drip down the sides. Let the glaze set before slicing and serving.