Preheat oven to 325℉. Grease two 9-inch round cake pans and line with parchment.
In the bowl of a stand mixer fitted with the paddle attachment, beat butter and brown sugar on medium speed until creamy, 3 to 4 minutes. Scrape the sides of the bowl, then turn the mixer back on. Add eggs, one at a time, beating well after each addition. Beat in vanilla.
In a medium mixing bowl, whisk together flour, baking powder, salt, cinnamon, and baking soda. In a small bowl or glass measuring cup, whisk together the maple syrup and sour cream.
With mixer on low speed, gradually add flour mixture to butter mixture alternating with the maple syrup mixture. I like to add one-third the flour mixture followed by half the maple syrup mixture, then repeat again in that order until done. Now add pecans and mix until just combined. Use a spatula to give the batter one more fold to make sure the pecans are evenly incorporated.
Divide the batter evenly between the two prepared pans and smooth the tops. Bake until a cake tester or wooden skewer comes out clean when poked into the center of the cake, about 30 to 35 minutes. Let cakes cool completely in the pans.
Once cool, tip the first cake out of the pan and peel off the parchment. Set the bottom cake on a cake stand and dollop some buttercream on top. Use an offset spatula to spread the frosting into an even layer. If some is poking out that’s fine. Now tip the second cake out of its pan and peel off the parchment. Place it on top of the bottom layer, lining it up evenly.
Place large dollops of frosting and spread it from the center to the edges and tipping over the sides of the cake. Gently frost the sides top down, making sure the spatula doesn’t touch the cake itself (that gets your crumbies in your frosting). Add more frosting to the sides as necessary. I used ALL the frosting here. Use your spatula to smooth out the top and sides of the cake. Sprinkle sea salt on the top, then decorate with pecans - or not - as you prefer.