Preheat the oven to 350 degrees F. In a large mixing bowl, mix together dry ingredients - all purpose flour, baking powder, baking soda, cornstarch, kosher salt, and ground cardamom. Whisk together.
In a separate bowl or bowl of a stand mixer fitted with the paddle attachment, cream room temperature butter and vegetable oil together for 2 minutes, then add brown sugar and sugar and continue to mix for another 3 minutes. Then add vanilla extract and eggs, one at a time - making sure each egg is fully incorporated prior to adding the next. Then add sour cream and ½ cup of whole milk. Mix wet ingredients until fully combined.
Add half of the dry ingredients and mix together and then add remaining ½ cup of milk. Then add the remaining dry ingredients and mix until fully incorporated, about 15-20 seconds.
Spray 3 8-inch cake pan with nonstick cooking spray. Divide batter into 3 8-inch cake pans. Tap the cake pans onto the countertop to remove any air bubbles. Bake for 20-22 minutes, until a toothpick or wooden skewer comes out clean.
Allow the cakes to cool completely on a wire rack.
While the cakes are cooling, make the maple frosting. Add room temperature butter to a large bowl and beat on high speed until fluffy and lighter in color. Then add cream cheese and begin adding confectioners’ sugar 1 cup at a time, beginning on low speed and increasing to high speed once powdered sugar is fully incorporated.
Add maple extract, heavy cream, and kosher salt and continue to mix together until smooth.
Add one of the cakes to a cake stand and add about ¾ cup of buttercream to the top of the cake. Place the second cake on top and repeat.Place the third cake on top and add a thin layer of maple buttercream all around the cake. Allow the cake to set in the refrigerator for 30 minutes.
Add remaining buttercream to the rest of the cake.