Preheat the oven to 350°F (175°C) and prepare three 8-inch cake pans by greasing them with butter or oil and lining the bottoms with parchment paper.

In a large mixing bowl, whisk together the all-purpose flour, baking soda, and salt until well combined. Set aside.

In a stand mixer, cream together the unsalted butter and light brown sugar on medium speed until light and fluffy, about 3-4 minutes. Add in the eggs one at a time, mixing well.

Mix water and vinegar in a separate bowl. Gradually add the dry flour mixture and the water-vinegar mix to the butter and sugar mixture, alternating, mixing just until combined.

Divide the batter into two parts, leaving one part plain and mixing cocoa powder into the other. Pour vanilla batter into two prepared pans and chocolate batter in the remaining pan.

Bake layers for 19-22 minutes or until a toothpick comes out clean. Let cool in pans for 10 minutes before transferring to a wire rack.

Cool the layers completely on a wire rack.

Prepare the frosting by melting chocolate and butter until smooth. Mix in the chopped pecans and vanilla extract, then let cool slightly.

Brush the tops of the cooled cake layers with hazelnut liqueur. Begin assembling the cake by alternating layers of vanilla and chocolate with frosting in between.

Apply a thin crumb coat of frosting to the sides and top, then refrigerate for about 30 minutes before applying a thicker layer of frosting.

Garnish the top with additional chopped toasted pecans or hazelnuts. Let the cake set in the refrigerator for at least 3 hours before slicing.

Serve slices with whipped cream or vanilla ice cream.