Step 1: Prepare the Cake

1. Preheat Oven: Preheat your oven to 350°F (175°C). Grease and flour three 8-inch round cake pans.
2. Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
3. Cream Butter and Sugar: In a large mixing bowl, beat the softened butter and granulated sugar until light and fluffy, about 3–4 minutes.
4. Add Eggs and Vanilla: Add the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract.
5. Combine Mixtures: Gradually add the dry ingredients to the butter mixture, alternating with the buttermilk. Begin and end with the dry ingredients. Mix until just combined.
6. Fold in Pecans: Gently fold in the toasted pecans.
7. Bake: Divide the batter evenly among the prepared cake pans. Bake for 25–30 minutes, or until a toothpick inserted in the center comes out clean.
8. Cool: Let the cakes cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely.

Step 2: Make the Maple Frosting

1. Beat Butter: In a large mixing bowl, beat the softened butter until creamy and smooth.
2. Add Powdered Sugar: Gradually add the powdered sugar, mixing on low speed until combined.
3. Add Maple Syrup and Vanilla: Pour in the maple syrup and vanilla extract, and beat until smooth.
4. Adjust Consistency: Add heavy cream, one tablespoon at a time, until the frosting reaches your desired consistency.

Step 3: Make the Brown Sugar Crumble

1. Combine Ingredients: In a bowl, mix together the flour, brown sugar, melted butter, and toasted pecans until the mixture is crumbly.
2. Bake: Spread the crumble on a baking sheet lined with parchment paper and bake at 350°F (175°C) for 10–12 minutes, or until golden brown. Let cool completely.

Step 4: Assemble the Cake

1. Layer the Cake: Place one cake layer on a serving plate or cake stand. Spread a generous layer of maple frosting over the top.
2. Repeat Layers: Add the second layer and spread more frosting. Place the third layer on top and frost the entire cake, smoothing the sides and top.
3. Add the Crumble: Sprinkle the brown sugar crumble evenly over the top of the cake. Gently press some crumble along the edges for a decorative touch.