Preheat the oven to 350 degrees.
Whisk together the flour, sugar, cocoa powder, and baking soda in a bowl. Set aside.
Combine mayonnaise, milk, and vanilla extract in a stand mixer or use a bowl with a hand mixer. Beat together on a low speed for 30 seconds.
Slowly add in the dry mixture until everything is combined (about 15 seconds).
Grease a 9” round pan with extra butter and dust with flour. Spread the cake batter in the pan.
Bake for 30-35 minutes, or until a toothpick comes out clean.
Take the cake out of the oven and let it cool in the pan for 10 minutes before removing.
Remove to a cooling rack or a plate and let the cake sit for 1 hour to completely cool off.
Frost with the raspberry buttercream frosting and garnish with fresh raspberries and chocolate shavings, if desired.