Preheat oven to 350F.

Sift together flour, baking soda, sugar, and salt.

Make a well in the middle of powdered ingredients and put egg yolks, milk, oil, lemon zest and vanilla in.

Whisk together till all is incorporated and color is light yellow.

In a mixer, using whisk attachment, mix egg whites and cream of tartar.

Whip till stiff peaks 6 - 8 minutes.

Add ¼ egg whites to yolk mixture and stir until combined.

Gently fold in rest of egg whites. Fold till just incorporated.

Ladle in cake mix to each cavity. Don’t fill to the top as the cakes will rise.

Bake 16 - 18 minutes, until cakes are a little golden.

Remove from pan and place on cooling rack.

Store in airtight container at room temperature.