Preheat the oven to 350°F (175°C). Grease and flour a 9x13 inch cake pan.

Cream together 1 cup of softened unsalted butter and 2 cups of granulated sugar until light and fluffy.

Add 4 large eggs one at a time, mixing well after each addition, then fold in 1 teaspoon of vanilla extract.

In a separate bowl, whisk together 2 1/2 cups of all-purpose flour, 2 teaspoons of baking powder, 1/2 teaspoon of baking soda, and 1/2 teaspoon of salt.

Alternately add the dry ingredients and 1 cup of buttermilk to the wet ingredients, mixing on low speed until combined.

Fold in 1 cup of chopped, toasted pecans into the batter using a spatula.

Pour the batter into your prepared cake pan and smooth the top evenly.

Bake the cake for 30-35 minutes, until a toothpick inserted comes out clean.

Let the cake cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely.

In a saucepan, combine 1/2 cup of unsalted butter, 1 cup of brown sugar, and 3 tablespoons of milk over medium heat until it boils.

Whisk in 2 cups of powdered sugar and 1 teaspoon of vanilla extract until smooth.

Spread the warm frosting over the cooled cake and sprinkle with additional toasted pecans for decoration.

Slice and serve, optionally with vanilla ice cream.