Preheat oven to 350 F.
Spray a half sheet pan ( 18"x13"x1") with baking spray and line the bottom with parchment paper, spray the parchment paper and dust the pan with a little flour, tapping out any excess.
In a large bowl, mix dry ingredients: flour, baking powder, baking soda and salt.
In the bowl of a stand mixer fitted with the wire whisk, on medium speed beat butter, gradually add the sugar and beat until light and fluffy. Add the eggs one at a time and whisk until combined.
Add oil and vanilla extract, mix until fully combined, pause to scrape the sides of the bowl with a spatula.
Add the dry ingredients followed by the kefir. Mix until blended, taking care not to over beat.
Manually, with a spatula, fold in the M&M's White Strawberry Shortcake.
Transfer batter to the baking sheet, spread evenly and bake for 30-35 minutes. A toothpick inserted in the middle of the cake must come out clean.
Remove cake from oven and let it cool in the pan for 15 minutes, after that, with the help of the parchment paper, slide the cake from the pan, to the counter to cool completely.
Once completely cooled, using a round stainless steel cake ring, cut two or three round cakes, in dependence how big the ring is.