Preheat the oven to 350°F (175°C). Grease a 9-inch round cake pan or 8-inch square pan. Line the bottom with parchment paper.
Whisk together the flour, baking powder, baking soda, salt, and cinnamon. Set aside.
In a large bowl, beat the butter, brown sugar, and sugar with an electric mixer on medium speed until light and fluffy (3-4 minutes).
Add the eggs one at a time, mixing well after each addition. Mix in the vanilla and maple extracts. Then mix in the sour cream and maple syrup. (The batter may look slightly curdled.)
With the mixer on low speed, gradually add the flour mixture and mix just until combined. Fold in the pecans if using.
Spread the batter evenly in the prepared pan.
Bake 30 to 35 minutes, or until a pick inserted into the center comes out clean. Cool completely before frosting.
To make the frosting:
Using an electric mixer on medium speed, beat the butter and salt until creamy.
Add 2 1/2 cups confectioners’ sugar along with the maple syrup and maple extract. Mix until combined. If needed, add more cream for a soft, spreadable consistency, or add more of the sugar for structure. (Different brands of syrup can loosen the frosting in varying ways.)
Increase the mixer speed to medium-high at the end for 1 to 2 minutes to create a fluffy texture.
Spread over the cooled cake and garnish with toasted pecans if desired.
Notes
Pecans: These are optional or can be substituted with walnuts. Toasting them first brings out their flavor and makes them a delicious complement to the maple.
Maple extract: Maple extract helps boost the flavor of the pure maple syrup so it comes through clearly in the finished cake. If you don't have maple extract, you can omit it. The maple flavor will be more subtle, but I don't recommend adding extra maple syrup in its place, as that can affect the texture of both the cake and frosting.
Storage: Keep in an airtight container at room temperature up to 2 days or in the refrigerator up to 5 days.