Preheat your oven to 350°F and line a 9x13-inch baking pan with parchment paper, allowing some overhang on the sides for easy removal. This will be helpful later when lifting the cooled blondies out. Bring your eggs to room temperature—this helps them incorporate more smoothly into the butter mixture and creates a lighter, fluffier texture. Measure out all your ingredients and have them ready before you begin mixing.

In a large mixing bowl, beat together the softened unsalted butter, granulated sugar, and packed light brown sugar for about 3 minutes on medium-high speed until the mixture is light, fluffy, and pale in color. This creaming process incorporates air into the batter, which helps create the tender, cake-like texture these blondies are known for. I find that Kerrygold butter adds a wonderful richness that elevates these treats.

Add the room temperature eggs one at a time to the creamed butter mixture, beating well after each addition. Pour in the pure vanilla extract and beat until everything is well combined and the mixture looks smooth and slightly thickened. Scrape down the sides of the bowl to ensure no unmixed ingredients remain.

In a separate medium bowl, whisk together the all-purpose flour, baking powder, cream of tartar, and salt. Add this dry ingredient mixture to the wet mixture from Step 3, beating on low speed just until combined—don't overmix, as this can lead to tough blondies. Fold in the 1 cup of white chocolate morsels with a spatula until evenly distributed throughout the dough.

Spoon half of the dough into your prepared pan and gently press it into an even layer. In a small bowl, mix together the 1/4 cup granulated sugar and freshly ground cinnamon. I prefer freshly ground cinnamon here—it has a brighter, more vibrant flavor than pre-ground. Sprinkle about three-quarters of this cinnamon-sugar mixture over the bottom layer, saving roughly 1 teaspoon for the top. Flatten the remaining dough between your hands or with a spatula and carefully place it over the cinnamon layer. Press the remaining 1 tablespoon of white chocolate morsels into the top of the dough, then sprinkle with the reserved cinnamon-sugar mixture.

Place the pan in your preheated 350°F oven and bake for 32–35 minutes, until the top is golden brown and a toothpick inserted into the center comes out with just a few moist crumbs. The blondies will continue to set as they cool, so avoid overbaking or they'll become dry.

Allow the blondies to cool in the pan for about 1 hour until completely cool. Using the parchment paper overhang, carefully lift the entire block out of the pan and place it on a cutting board. Cut into even squares using a sharp knife—I like to wipe the blade between cuts for clean edges. Serve at room temperature and store in an airtight container for up to 3 days.