Heat oven to 375°F/200°C. Grease a 9- or 8-inch/21-centimeter loaf pan with butter, line it with a parchment paper sling and butter the paper. Set the pan aside.

Place butter, sugar, lemon zest, and vanilla extract in the bowl of a stand mixer fitted with the paddle attachment. Beat on high speed for 3 to 4 minutes, until light, then lower speed to medium. Add eggs in three additions, scraping down the sides of the bowl a few times as necessary. The mix may split a little but don’t worry: It’ll come back together once you add the dry ingredients.

In a separate bowl, whisk together flour, baking powder, and salt. With the stand mixer on low, add the dry ingredients in three additions, mixing just until no white specks remain. Fold in about ½ of the blueberries by hand, then scoop batter into the prepared loaf pan.

Bake for 15 minutes, then sprinkle the remaining blueberries over the top of the cake. Return to the oven for another 10 minutes, until cake is golden brown but still not cooked through. Cover loosely with foil and continue to bake for another 30 to 40 minutes (less for a 9-inch pan, more for an 8-inch pan), or until risen and cooked through, and a knife inserted into the middle of the cake comes out clean. Remove from oven and set aside in its pan to cool for 10 minutes before removing cake from pan and placing on a wire rack to cool completely.

OPTIONAL: When cake is cool, make the icing: Add lemon juice and icing sugar to a bowl and whisk together until smooth, adding a bit more juice, if necessary, just until the icing moves when you tilt the bowl. Pour over the cake and gently spread out. The blueberries on the top of the cake may bleed into the icing a little, but this will add to the look. Let icing set (about 30 minutes), slice and serve.