Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper or silicone baking mats.
In a bowl of a stand size mixer fitted with the paddle attachment, beat the cold cubed butter, white sugar, brown sugar, and lemon zest on medium high speed for 3-4 minutes. The mixture is ready for the next step when it looks soft, smooth and fluffy. Make sure you do this step, if you don’t do it right, you will have flat cookies.
*you can use a hand mixer cream the ingredients together, it will take longer than 3-4 minutes. You want the butter and sugar to be well combined and light and fluffy.
With a rubber spatula, scrape down the sides and bottom of the bowl, add the egg and beat on medium speed for 30 seconds. Scrape down the sides and egg yolk to the mixture beating for 30 more seconds. Add the vanilla extract and mix low.
Add the all-purpose flour and baking powder with the mixer on the lowest speed mix in the dry ingredients to the wet mixture, mixing until just combined. Be careful not to overmix.
With a rubber spatula, gently fold in the fresh blueberries until evenly distributed throughout the dough. You want to be so gently that you don’t pop the blueberries into the dough.
Note: you want to use 4 tablespoon cookie scoop. To create the marble affect with the blueberry jam, place 2-3 dollops of jam on top of the dough, and gently swirl with a toothpick.
Scoop out the 3 dough balls and place it on the prepared baking sheets, leaving some space between each cookie.
Add 2-3 more dollops of jam to the top of the cookie dough and repeat the process. Do this until you have scoop out all 10 of the cookie dough balls.
You can fit all 10 cookies on the sheet. Once done, place in the fridge while you make the topping.
To make the crumb topping, combine the cold butter, light brown sugar, cinnamon, and all-purpose flour in a small bowl. Use your fingers or a pastry cutter to cut the butter into the dry ingredients until the mixture resembles coarse crumbs.
Sprinkle the crumb topping evenly over each cookie, pressing it lightly into the dough. Cover the dough ball entirely
Add a few blueberries to garnish the top of each cookie dough ball to get it a finish look
Bake the cookies in the preheated oven for 12-15 minutes, or until the edges are golden brown and the tops are set.
Allow the cookies to cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.
Once cooled, enjoy your delicious lemon blueberry muffin cookies as a delightful treat or snack! Store any leftovers in an airtight container at room temperature for up to several days. Enjoy!