• Heat oven to 350° with rack set in lower-middle position.

Cake

• Whisk together 1 tablespoon cocoa powder and 1 tablespoon melted butter to make a paste. Thoroughly coat the interior of a 12-cup bundt pan with cocoa-butter paste using a pastry brush.
• Combine remaining ¾ cup cocoa powder and chocolate in a medium bowl. Microwave coffee in a glass measuring cup until simmering, then pour over chocolate mixture, cover, and let stand 5 minutes. Whisk chocolate mixture until smooth, let cool slightly, then whisk in sour cream.
• In a separate bowl, whisk together flour, salt, ground ginger (if using), and baking soda.
• In a stand mixer fitted with the paddle attachment, beat butter, sugar, and vanilla on medium-high speed until pale and fluffy, about 4 minutes.
• Add eggs, one at a time, beating after each addition. Reduce speed to low and add flour in three additions, alternating with chocolate-sour cream mixture, ending with flour, scraping down sides of bowl as needed.
• Add candied ginger (if using) and mix just until incorporated.
• Transfer batter to prepared pan and smooth top with a spatula. Bake cake until a skewer inserted in the center comes out with a few crumbs, 45–50 minutes.
• Let cake cool in pan 10 minutes, then invert onto a wire rack and cool completely.

Glaze

• Place chocolate in a medium bowl. Bring cream to a simmer in a small saucepan then pour over chocolate and let stand 2 minutes. Add butter and whisk until combined, let cool slightly.
• Drizzle glaze over cake and sprinkle candied ginger (if using); let glaze set before serving.