In a small bowl, whisk together the blueberry Greek yogurt, whipped cream cheese, lemon zest, lemon juice, and simple syrup until smooth. Transfer the mixture to a zip-top bag and snip a small corner off the bag.
In a medium bowl, whisk together the eggs, half & half, vanilla extract, and cinnamon until fully combined.
Using a serrated knife, carefully cut a pocket into the side of each brioche slice, being careful not to cut all the way through. Pipe the blueberry cream cheese mixture into each pocket until filled.
Heat your griddle to medium (about 350°F if using the E-Series). Add the butter and allow it to melt and coat the surface.
Dip each stuffed slice into the custard mixture, making sure both sides are fully coated. Place on the griddle and cook for 3–5 minutes per side, or until golden brown, slightly crispy, and cooked through.
Transfer to a serving platter and dust with powdered sugar.