Preheat oven, prepare cake pans: Preheat oven to 350 degrees. Butter and line 3 9-inch round cake pans with parchment paper, butter parchment paper and lightly dust pans with flour shaking out excess. Set pans aside.
Saute pecans in butter: Melt 3 Tbsp butter in a skillet over medium heat. Once melted add pecans and cook until slightly browned and fragrant, tossing frequently, about 4 minutes. Pour and spread onto a sheet of parchment paper or a baking sheet. Set aside and allow to cool.
Mix liquids together: stir together buttermilk and milk, set aside.
Whisk dry ingredients: In a mixing bowl whisk together flour, baking powder, baking soda and salt for 20 seconds, set aside.
Cream butter and sugar, then eggs and vanilla: In the bowl of an electric stand mixer fitted with the paddle attachment whip together butter and sugar until very pale and fluffy. Mix in eggs one at a time adding in vanilla with last egg.
Mix in flour alternating with milk mixture: Add 1/3 of the flour mixture to the butter mixture and mix on low speed just until combined, then add in 1/2 of the buttermilk mixture and mix just until combined then repeat process once more. Finish by mixing in remaining 1/3 of the flour mixture and mix just until nearly combined.
Fold in pecans: Remove bowl from stand mixer and fold in pecans (fold batter until there's not longer streaks in the batter). Divide batter evenly among prepared baking pans.
Bake until set: Bake in preheated oven 23 - 28 minutes until toothpick inserted into center of cake comes out clean. Remove from oven and cool in cake pans 10 minutes.
Let cake cool completely on wire rack: Run a knife around edges of cake to ensure the are loosened and invert onto wire racks to cool completely. Once cool frost with cream cheese frosting and top with pecans.