Heat oven to 325°F. Grease and flour a bundt pan; set aside.: As the oven warms, you will notice a gentle hum and eventual steady glow from the heating element, which creates the even environment this dense cake needs to rise slowly without cracking excessively. Greasing and flouring the bundt pan ensures the cake releases cleanly, and you should feel the smooth texture of your pan surface as you coat it. The why here is simple, the pan coating prevents sticking and protects the cake’s shape. A common mistake is insufficient greasing which causes the cake to tear when inverted, so take a minute to get full coverage and tap out excess flour.
In a large bowl, combine sugar and butter at medium speed, scraping the bowl often, until creamy. Continue beating, adding 1 egg at a time, until well mixed.: You will hear the mixer change tone as the butter and sugar transform from grainy to silky and pale. The aroma will become buttery and slightly sweet, and the texture should look light and aerated, which traps tiny bubbles that expand in the oven to create structure. Adding eggs one at a time maintains a stable emulsion, avoiding separation. If you rush this stage the batter will not incorporate air properly, so resist the urge to speed up or skip scraping down the bowl edges.
In a medium bowl, mix the flour, baking powder, baking soda, and salt.: When you whisk these dry components together you create a uniform distribution of leavening so the cake rises evenly. The visual cue is a homogenous pale mixture with no pockets of baking powder or baking soda. This is important because uneven leavening results in uneven texture or odd flavor pockets. A typical error is adding the leaveners directly without sifting or mixing, which risks having concentrated pockets.
On low speed alternately add the flour mixture and the buttermilk and scrape the bowl often, until well mixed. Add lemon zest and 1 tablespoon of lemon juice. Continue beating until combined.: At this stage you will notice a rhythm, folding in dry, then liquid, so the batter becomes smooth and glossy. The buttermilk contributes to a tender crumb and reacts slightly with the leaveners, while the alternating additions prevent overworking gluten. Once the lemon zest and lemon juice are folded in you will get a bright citrus perfume. If you overmix after adding the flour the crumb will toughen, so mix just until no streaks of flour remain.
Spoon batter into prepared pan. Bake for 55-65 minutes or until a toothpick inserted in the center comes out clean. Cool in pan for 10 minutes. Invert onto a serving plate; cool completely.: As the loaf bakes the top will slowly turn golden and a clean, sweet aroma will fill the kitchen. The batter’s surface will spring back gently to the touch when it is near done, and a toothpick should come out clean or with a few moist crumbs. Cooling in the pan for 10 minutes allows the structure to set so the loaf releases without collapsing, and inverting it onto a rack finishes the cooling. Cutting into the cake too early is the usual misstep that leads to a gummy interior, so be patient and let it cool fully before glazing.
For the glaze, stir powdered sugar, 2 teaspoons of lemon juice, and enough milk for desired glazing consistency in a small bowl. Drizzle over the cooled cake.: The glaze should be mixed until smooth, glossy, and just pourable, and when you spoon it over the top it should form thin ribbons that slowly melt into the surface. The aroma of fresh lemon juice will lift the sweetness, and adjusting the milk a drop at a time lets you control how far the glaze runs. Avoid making the glaze too thin which will pool at the base and hide the cake’s top, or too thick which will crack when sliced.