Step 1: Preheat and Prepare
Preheat your oven to 350°F (175°C). Grease two 8-inch or 9-inch round cake pans with butter or baking spray, then line the bottoms with parchment paper rounds. Grease the parchment too for double insurance against sticking. Set aside.

Step 2: Toast the Walnuts
In a dry skillet over medium heat, toast the chopped walnuts for 3-5 minutes, stirring frequently, until fragrant and lightly golden. Watch carefully—they can burn quickly! Transfer to a plate to cool completely. Toasting brings out the rich, nutty flavor and removes any bitterness.

Step 3: Mix Dry Ingredients
In a large bowl, whisk together the flour, baking powder, baking soda, and salt until well combined. Whisking ensures all leavening agents are evenly distributed for uniform rising. Set aside.

Step 4: Cream Butter and Sugars
In your stand mixer bowl (or large bowl with hand mixer), beat the room-temperature butter on medium speed for 1 minute until smooth. Add both sugars and beat on medium-high speed for 3-4 minutes until light, fluffy, and pale in color. This creaming process incorporates air that creates tender texture. Scrape down the bowl sides occasionally.

Step 5: Add Eggs and Flavorings
Add eggs one at a time, beating well after each addition (about 30 seconds per egg). The mixture may look slightly curdled—that’s normal! Add the maple syrup and vanilla extract, beating until combined. The mixture will be quite liquidy at this point—don’t worry!

Step 6: Combine Wet Ingredients
In a separate bowl or large measuring cup, whisk together the sour cream and milk until smooth. This makes it easier to incorporate into the batter.

Step 7: Alternate Adding Dry and Wet
With mixer on low speed, add the flour mixture in three additions, alternating with the sour cream mixture in two additions. Start and end with flour: flour, sour cream mixture, flour, sour cream mixture, flour. Mix just until each addition is incorporated—don’t overmix! Scrape down the bowl to catch any unmixed bits.

Step 8: Fold in Walnuts
Using a rubber spatula, gently fold the cooled toasted walnuts into the batter by hand until evenly distributed. The batter should be thick, smooth, and studded with walnut pieces.

Step 9: Bake the Cakes
Divide the batter evenly between your prepared pans (use a kitchen scale for precision). Smooth the tops with a spatula. Bake for 28-32 minutes, or until a toothpick inserted in the center comes out clean or with just a few moist crumbs, and the tops spring back when lightly touched.

Step 10: Cool Completely
Let cakes cool in the pans on wire racks for 10-15 minutes. This brief rest helps them set. Then carefully turn them out onto wire racks, remove parchment, and let cool completely (at least 1 hour) before frosting. Frosting warm cakes will cause melting disasters!

Step 11: Make Maple Buttercream
In a clean mixing bowl, beat room-temperature butter on medium speed for 2 minutes until creamy. Add 3 cups of powdered sugar, maple syrup, vanilla, and salt. Beat on low until sugar is incorporated, then increase to medium-high and beat for 3 minutes until fluffy. Add cream 1 tablespoon at a time until you reach desired consistency. Add more powdered sugar if too thin, more cream if too thick. Taste and adjust maple flavor if desired.

Step 12: Assemble and Frost
If your cake layers have domed tops, use a serrated knife to level them. Place first layer on a cake stand or plate, spread about 1 cup of frosting evenly on top. Add second layer and frost the top and sides of the entire cake. Use an offset spatula for smooth, professional results. A thin “crumb coat” first (refrigerate 15 minutes to set) then final frosting layer gives the cleanest finish.

Step 13: Decorate and Serve
Decorate with additional toasted walnut halves or pieces on top if desired. Refrigerate for 30 minutes to set the frosting before slicing for cleanest cuts. Slice, serve, and enjoy the compliments!