Cream the butter, cream cheese, and sugars. Beat the butter, cream cheese, granulated sugar, and brown sugar together on medium speed for about 2 minutes, until pale and fluffy. Scrape the sides of the bowl halfway through — cream cheese likes to hide down there.
Add the egg, yolk, vanilla, and lemon zest. Beat in the egg and egg yolk one at a time, then mix in the vanilla and lemon zest until just combined. The extra yolk, instead of a second whole egg, keeps the dough rich without adding too much liquid.
Whisk the dry ingredients separately. In a second bowl, whisk together the flour, baking soda, baking powder, and salt. Whisking them first spreads the leaveners evenly through the flour, so you don’t end up with one cookie that puffs oddly.
Bring the dough together. With the mixer on low, add the dry ingredients in two additions, mixing just until no dry streaks remain. Stop as soon as the flour disappears overmixing here is the fastest route to a tough, dense cookie.
Fold in the blueberries by hand. Switch to a spatula and gently fold in the blueberries; a mixer paddle will crush them before you’ve scooped a single cookie. The dough should look thick and a little sticky.
Scoop, coat, and freeze don’t skip this part. Portion the dough into 24 balls with your cookie scoop. Roll each one in the graham cracker crumb-sugar mixture, arrange on a parchment-lined baking sheet, and freeze for 30–45 minutes. (Pro tip: freeze the whole tray, not a bowl of dough pre-portioned balls chill evenly and go straight into the oven.) This is the step most recipes leave out, and it’s exactly why so many blueberry cookies turn out thin and streaked purple: cold butter and cold berries hold their shape long enough for the outside of the cookie to set before the berries burst and the butter fully melts. Skip the freezer, and you’re baking a puddle instead of a cookie.
Bake. Preheat your oven to 350°F (175°C) while the dough chills. Bake the frozen dough balls for 12–13 minutes, until the edges are set and lightly golden but the centers still look slightly underdone they’ll firm up as they cool.
Cool, then glaze if you’re using it. Let the cookies rest on the baking sheet for 5 minutes before moving them to a wire rack; they’re fragile straight out of the oven. Once fully cool, whisk together the powdered sugar and lemon juice (or milk) and drizzle it over the top.
