Preheat oven to 350°F (175°C). Grease cake pans and line with parchment if using.
In a skillet, toast pecans with 1 tablespoon butter over medium heat until fragrant, about 5–7 minutes. Set aside.
Brown the remaining butter in a saucepan until golden and aromatic. Let cool to room temperature.
In a medium bowl, whisk flour, baking powder, baking soda, and salt.
In a large bowl, beat browned butter with both sugars until fluffy. Add eggs one at a time, mixing well. Add vanilla.
Mix in the dry ingredients alternately with buttermilk or sour cream. Fold in most of the toasted pecans, saving some for topping.
Pour into pans and bake for 25–30 minutes (round pans) or 35–40 minutes (9×13 pan), until a toothpick comes out clean. Cool completely.
For frosting: beat butter and cream cheese until smooth. Add powdered sugar, vanilla, and milk until desired texture is reached.
Frost cooled cake and top with reserved pecans.