Preparation

• Preheat the oven to 350°F. Butter and flour a 12-cup Bundt cake pan.
• In a medium bowl, whisk together the flour, cardamom, baking powder, salt, and baking soda.
• In a large bowl, combine the espresso powder and vanilla. Add the dark brown sugar, granulated sugar, and butter, and beat with an electric mixer on medium speed until fluffy, about 4 minutes. Beat in the eggs, 1 at a time, until well combined.
• Reduce the speed to low and alternate adding the flour mixture and the sour cream, starting and ending with the flour mixture, and mix until just combined. Transfer the batter to the prepared pan and smooth the top.
• Bake until puffed and set and a skewer inserted into the center comes out with moist crumbs attached, 50 to 55 minutes. Transfer the cake to a wire rack to cool in the pan for about 15 minutes, then flip the cake onto the rack to cool completely.

Prepare the glaze

• In a small saucepan, heat the condensed milk, butter, egg yolk, salt, and espresso powder over medium-low heat, stirring constantly, until thickened, about 4 minutes. Drizzle the warm glaze over the cooled cake. Top with the walnuts. Serve warm or at room temperature.