Make cake: Heat oven to 350°F. Grease 91/2-inch (10-cup) Bundt pan with melted butter, then sprinkle with granulated sugar, tapping to coat, making sure to get into all ridges.
In medium bowl, whisk together flour, baking soda, and spices.
In stand mixer, beat butter, granulated sugar, and grated ginger on medium-high until light and fluffy, 3 to 4 minutes. Beat in eggs 1 at a time, scraping bowl after each addition. Beat in vanilla.
In 4-cup measuring cup or medium bowl, stir molasses into 1 cup hot water. With mixer on low, add flour mixture to butter mixture in 3 increments, alternating with molasses mixture, until just combined.
Pour batter into prepared pan and smooth top. Bake until toothpick inserted into center comes out clean, 45 to 55 minutes. Let cool in pan on wire rack 20 minutes. Using small offset spatula, gently loosen edges and invert onto rack. Let cool completely.
Meanwhile, make glaze: In medium bowl, whisk together confectioners’ sugar, sour cream, heavy cream, vanilla, and salt, adding more confectioners’ sugar or heavy cream if needed, until thick and pourable.
Pour glaze over cake and garnish with pomegranate arils and freshly cracked black pepper. Let set 15 minutes before serving.