Preheat your oven to 325°F (163°C). Don't grease your chiffon cake pan—this cake needs to climb those sides like it’s training for a cake marathon.
In a clean bowl, whip the egg whites and cream of tartar until foamy. Gradually add half of the sugar while beating until stiff peaks form. Set aside, and give yourself a pat on the back—you just made meringue.
In another bowl, whisk the egg yolks, remaining sugar, oil, milk, and vanilla until well combined. Sift in the cake flour, baking powder, and salt, then whisk until smooth. No lumps—we’re not making blueberry dumplings here.
Gently fold the meringue into the yolk batter in thirds. Be gentle! Think of it like convincing a shy cat to come out from under the bed.
Toss the blueberries in a little flour (to stop them from sinking), then fold them into the batter.
Pour the batter into the ungreased pan. Smooth the top. Bake for 45–50 minutes or until the top springs back when gently poked.
Invert the cake (pan and all) onto a bottle or funnel and let it hang upside-down like a bat until completely cool.
Once cool, run a knife around the edges to release, then serve it up plain, dusted with powdered sugar, or with a big dollop of whipped cream.