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

In a mixing bowl, combine the butter pecan cake mix, water, vegetable oil, and eggs.

Using an electric mixer, beat on medium speed for 2 minutes until well combined.

Fold in the chopped pecans if using.

Pour the batter into the prepared baking pan.

Bake in the preheated oven for 25-30 minutes or until a toothpick inserted in the center comes out clean.

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

In a mixing bowl, beat the softened butter until creamy.

Gradually add the powdered sugar, mixing on low speed until combined.

Add the milk and vanilla extract, then beat on high speed for 3-4 minutes until fluffy.

Once the cake is completely cool, spread the frosting evenly over the top.

Garnish with additional chopped pecans if desired.

Slice and serve.