Preheat oven to 350 degrees.
Line a square baking pan with parchment paper.
In a stand mixer add the butter and the sweeteners.
Mix well.
Add the eggs and vanilla and beat, scraping sides of bowl to incorporate all.
Add 1 Tbsp apple cider vinegar to 1 cup measuring cup.
Fill with milk.
Add baking powder and gf flour blend to bowl and mix on low speed.
Add sour milk and mix on low.
Increase speed to medium and beat until batter is light and fluffy.
Pour batter into lined pan.
Bake for 28-30 minutes.
Remove and cool.
Enjoy!