Set your oven to 350°F (175°C). Grease a 9×5-inch loaf pan with butter or cooking spray, then line it with parchment paper so the cake will lift out easily later.

In a medium bowl, whisk together the flour, baking soda, and salt. Set this mixture aside for later.

In a large bowl, use an electric mixer to beat the softened butter and granulated sugar together. Keep mixing for about 3–4 minutes, until the mixture is pale and fluffy.

Beat in the eggs, one at a time, making sure each is fully incorporated before adding the next. Then add the lemon zest and lemon juice, mixing until everything is combined.

Add the flour mixture and buttermilk to the butter mixture in alternating batches, starting and ending with the flour. Mix gently after each addition, just until combined—don’t overmix.

Pour the batter into the prepared loaf pan and smooth the top with a spatula. Bake in the preheated oven for 65–75 minutes, or until a toothpick inserted in the center comes out clean. Let the cake cool in the pan for 10–15 minutes, then transfer it to a wire rack to cool completely.

In a small saucepan over low heat, melt the butter. Stir in the powdered sugar, sweetened condensed milk, and lemon juice until smooth. Remove from heat and let it cool slightly.

When the cake is completely cooled, pour the lemon icing evenly over the top. Let it set for at least 15–20 minutes before slicing and serving.