Step 1: Preheat your oven to 350°F (175°C). Generously grease and flour a 10-12 cup Bundt pan, ensuring all crevices are coated. Set aside.
Step 2: In a medium bowl, whisk together the 2 ¾ cups flour, baking powder, baking soda, and salt. 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).
Step 4: Add the eggs one at a time, beating well after each addition. Mix in the orange zest and vanilla extract.
Step 5: Gradually add the dry ingredients to the wet ingredients in three additions, alternating with the buttermilk in two additions, beginning and ending with the dry ingredients. Mix on low speed until just combined. Do not overmix.
Step 6: In a small bowl, toss the cranberries with the remaining 1 tablespoon of flour. This helps prevent them from sinking to the bottom of the cake.
Step 7: Gently fold the floured cranberries into the cake batter.
Step 8: Pour the batter evenly into the prepared Bundt pan. Bake for 50-60 minutes, or until a wooden skewer inserted into the center comes out clean.
Step 9: Let the cake cool in the Bundt pan on a wire rack for 10-15 minutes. Then, carefully invert the cake onto the wire rack to cool completely.
Step 10: While the cake cools, prepare the glaze. In a medium bowl, whisk together the powdered sugar, orange juice, and Grand Marnier (if using) until smooth. Adjust consistency by adding more powdered sugar for a thicker glaze or more orange juice for a thinner glaze.
Step 11: Once the cake is completely cool, drizzle the orange glaze over the top, allowing it to drip down the sides. Serve immediately or store.