Heat the oven to 350 F. Grease and line two 8-inch round cake pans, then whisk together the flour, baking powder, baking soda and salt.
Beat the butter and brown sugar until light and creamy. Beat in the maple syrup, followed by the eggs one at a time and the vanilla.
Mix in the sour cream. Add the dry ingredients in three additions, alternating with the milk, and mix only until the batter is smooth.
Divide the batter between the pans and bake for 27 to 32 minutes, until the centers spring back when lightly pressed. Cool in the pans for 15 minutes, then turn out and cool completely.
Cook the pecans, maple syrup, butter and salt in a skillet over medium heat for 4 to 5 minutes, stirring until the syrup coats the pecans. Spread them on parchment paper and cool completely.
Beat the butter for the frosting until creamy. Gradually beat in the powdered sugar, maple syrup, heavy cream, vanilla and fine sea salt until fluffy and spreadable.
Fill and cover the cooled cake layers with maple frosting. Decorate with the maple pecans and finish with the flaky sea salt.