Preheat oven to 350°F (177°C). Line a 9-inch square baking pan with parchment paper, leaving an overhang on the sides to easily lift the bars out of the pan. Set aside.

In a large bowl using a handheld or stand mixer fitted with a paddle attachment, beat the butter, granulated sugar, and brown sugar together on medium-high speed until fluffy and light in color, about 3 minutes. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Add the eggs and vanilla extract, and then beat on high speed until combined. Scrape down the sides and bottom of the bowl as needed.

In a separate bowl, whisk the flour, baking powder, cream of tartar, and salt together until combined. Pour into the wet ingredients, then beat on low speed until combined. Beat in 1 cup (180g) white chocolate morsels. The cookie dough will be thick and sticky.

Spoon half the dough into the prepared pan and press/spread it into an even layer. If using your hands to press it down, flour your hands first. This is a sticky dough.

In a small bowl, mix together the sugar and cinnamon for the filling. Sprinkle the cinnamon-sugar on top of the bottom layer, reserving 1 teaspoon for the top. Flour your hands, take a handful of the remaining dough and press it between your hands to flatten it. Place it on top of the cinnamon swirl layer. Repeat with the remaining dough, a handful at a time, re-flouring your hands as often as needed. Lay the “shingles” of dough gently next to one another, sort of like puzzle pieces. It’s ok if some of the cinnamon-sugar layer is exposed/not covered. Press the remaining white chocolate morsels into the top, and then lightly sprinkle the surface with remaining cinnamon-sugar.

Bake until golden brown, about 32–35 minutes, or until a toothpick inserted in the center comes out mostly clean with only a few moist crumbs. Avoid over-baking.

Cool completely in the pan set on a cooling rack, about 1 hour. Once cool, lift the bars out of the pan using the parchment overhang on the sides, and cut into squares.

Cover and store leftover bars at room temperature for up to 1 week.