Preheat oven to 350 F.
Sift cake flour, baking powder, baking soda and fine sea salt into a bowl. Set aside.
In a separate bowl combine the butter and sugar. You will cream on medium high until light and fluffy - about 3 minutes.
Slowly drizzle in the oil while mixing on low. Increase to medium low once all oil has been added and mix until smooth.
Add one egg at a time and mix on medium until combined - about 1 minute.
Add the egg white at the end and mix on medium for about 1 minute.
Scrape down bowl and add vanilla. Mix on low until combined.
Add ⅓ of dry ingredients to wet ingredients. Mix on low until barely combined.
Add ½ of kefir to batter and mix on low until just combined. Repeat step 7 (add ⅓ dry) and step 8 (add other ½ of kefir) and ensure it's mixed until just combined.
Add remaining dry ingredients and mix on low until only some flour streaks remain.
Add sprinkles to batter, and use a rubber spatula to continue folding in the remaining dry ingredients and folding in the sprinkles. Fold in gently. Stop folding in sprinkles once no flour remains to be dispersed - you do not want to over mix the batter.
Prepare a 8x8 baking pan. Butter pan along bottom and up the edges. Then sprinkle in about 1 tsp of flour and shake it about to coat the butter. You can also opt to use parchment paper in place of this method.
Pour batter into butter and floured baking pan. Use a rubber spatula to gently smooth out butter evenly. You can gently tap the pan on the counter to help settle it.
Bake at 350 F for 40 minutes, or until a tooth pick comes out with a few crumbs from the center.
Place on wire rack to cool. Remove from baking pan after about 15 minutes and allow to continue cooling.