Preheat oven to 400 degrees F.: As the oven warms up you should hear the faint hum of the heating element and feel a rush of dry heat when you open the door, which primes the pan for an initial burst of oven spring. This high start helps set the outer structure quickly, creating a fine crust that then allows the center to rise evenly. A common mistake is not preheating thoroughly, which can lead to uneven rise and longer bake times, so wait until the oven reaches a steady temperature before slipping the pan in.
In the bowl of a mixer, combine butter and sugar and beat well to combine (mixture will be fairly dry). Add in eggs and beat until well combined. Add sour cream, vanilla and lemon extracts, lemon zest and lemon juice and beat until well combined.: When you cream butter with sugar the texture should go from dense to slightly paler and smoother, with a soft, pillowy quality. As you add the eggs , the batter will gain sheen and loosen, and folding in sour cream and the citrus will introduce a fragrant lift. You should be able to smell the fragrant oils from the lemon zest and the sweet dairy notes from the sour cream . Avoid overbeating after adding flour later, because too much air and gluten development will toughen the crumb.
In a separate bowl combine flour, baking powder, baking soda and salt.: The dry mix should be uniform, with the pale dust of flour carrying tiny specks of leavening. Sifting or whisking aerates the mixture, which helps with a consistent texture. Mixing the leaveners and salt thoroughly avoids pockets of metallic or overly salty taste in the finished loaf. If you skip this step you risk uneven rise or sudden salty bites.
Add dry ingredients to wet ingredients and mix by hand until combined, but do not over mix. Toss blueberries in flour. Fold in blueberries into batter and mix until just combined. Do not overmix.: As you fold, pay attention to the batter's texture it should look slightly thick and ribbon like, not runny. The sound will be quiet, the batter moving slowly under the spoon. Tucking in the coated blueberries gently keeps them whole and avoids bruising; you should see flecks of color start to appear. Overmixing at this stage develops gluten and will give a chewy, dense loaf rather than tender cake like slices.
Pour batter into a 9×5 loaf pan that has been sprayed with non-stick cooking spray.: The batter will look glossy and speckled with blueberry pockets, and pouring it into the prepared pan should feel smooth and even. Using a spatula to level the top helps ensure uniform rise. If the pan is not prepared, the loaf can stick and tear when removed, so always grease or line the pan in advance.
Place in the oven and immediately reduce the temperature to 350 degrees F. Bake for approximately 45 minutes or until a cake tester inserted into the center of the loaf comes out clean. If loaf does not see done at 45 minutes but is browning, foil can be placed loosely over the top to allow more baking time without additional browning.: You will notice the aroma shift from raw batter to baked lemon and caramelized sugar around twenty minutes in, and the top will develop a golden tone. Reducing the temperature right away ensures the inside cooks more gently while preserving the top color. If the loaf browns too quickly, tenting with foil prevents burnt edges while the center finishes. A frequent issue is opening the oven too often, which drops temperature and lengthens baking time.
Remove from oven and allow to cool.: Cooling lets the interior set so the loaf slices cleanly; you might notice a subtle release of steam and a mellowing of the lemon scent as it rests. Leave it in the pan briefly to firm up, then transfer to a rack to cool more evenly. Slicing too hot will cause the crumb to compress and crumble, so patience here improves both texture and appearance.
Combine icing ingredients in a bowl and mix until combined. If a thicker icing is desired, add more powdered sugar, one tablespoon at a time. If a thinner icing is desired, add more milk, one teaspoon at a time.: The glaze should be glossy and pourable, with the tang of lemon juice balanced against the sweetness of powdered sugar . Stirring slowly helps dissolve any lumps and yields a smooth sheen. Adjusting texture carefully keeps the balance right; adding too much liquid at once will make the glaze run off and pool excessively.
Remove loaf from pan (it helps to run a knife around the edges first to make sure the loaf isn't sticking to the pan) and pour or spoon glaze over the top. Allow icing to set up for 20+ minutes, then slice and serve.: After unmolding, the warm loaf will accept the glaze more readily, allowing it to sink into tiny crevices and highlight the crumb. Let the glaze set until it becomes matte to avoid messy slices. A common trap is slicing before the glaze firms, which causes smearing and imperfect slices, so wait at least twenty minutes for a neat presentation.