Preheat the oven to 350°F. Grease and line an 8" round cake pan, set aside.
In a large mixing bowl combine the flour, baking powder, baking soda, cinnamon, and salt. Stir to combine.
In the bowl of a stand mixer or using an electric hand mixer, fitted with the paddle attachment, cream together the butter and sugar until light and fluffy, 2-3 minutes. Scrape down the sides of the bowl and add in the egg whites, beat on high until combined. Gently mix in the sour cream and vanilla.
With the mixer on low, slowly add in the dry ingredients, mix until incorporated. Add in the milk and mix again until combined.
Transfer batter to the prepared baking pan. Bake at 350 for 30-35 minutes until a toothpick inserted comes out clean. Allow to cool completely on a wire rack before frosting.
For the buttercream:
In the bowl of a stand mixer or using a hand mixer fitted with a paddle attachment, beat the softened butter on high for 2-3 minutes. Gradually add in the powdered sugar until fully combined. Add in the maple syrup, heavy cream, and vanilla extract, mix until fluffy.
Spread over the cooled cake. Slice and enjoy immediately or store any leftover cake in an airtight container in the fridge for 3-4 days.