My dad drinks instant coffee that he first "brews" in the microwave and then dilutes with overflowing spoonfuls of condensed milk. As a youthful coffee snob, I turned up my nose and insisted that what he drank wasn't real coffee. But as I've gotten older and become less of a jerk, I've realized that what's cool doesn't matter when it's delicious. My dad's morning cup, super sweet and creamy with just a hint of coffee, is damn good. And now, I always ask him to make me one, too. This glorious coffee-flavored Bundt is more sweet than coffee and has a gooey, condensed milk glaze that you may have to resist eating with a spoon.
Preheat the oven to 350 degrees 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 vanilla and espresso powder. 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, scraping down the sides of the bowl as necessary, 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 for about 15 minutes, then flip out 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.