Preheat the oven to 350 degrees. Butter two 9-by-2-inch round cake pans, and dust with flour. Tap out any excess flour, and set pans aside.
In the bowl of an electric mixer fitted with the paddle attachment, beat the butter until creamy. Add the maple syrup, and beat until combined. Add the eggs, and beat until combined.
In a large bowl, sift together the flour, baking powder, salt, and ginger. Add the flour mixture to the butter mixture, and beat to combine. Beat in the milk and vanilla extract until combined. Stir in 3/4 cup chopped walnuts.
Divide batter between the two prepared pans. Bake until golden and a cake tester inserted into the centers comes out clean, about 40 minutes; rotate the cake pans halfway through baking time to ensure even baking. Transfer pans to a wire rack to cool.
Turn out cakes, and place one, top-side up, on a cake stand or platter. Spread 1 1/2 cups maple buttercream frosting evenly over the top. Place the second cake layer on top, and spread the remaining 2 1/2 cups frosting around the sides and over the top. Using your hands, gently press the remaining 3/4 cup walnuts onto the sides of the cake.