Preheat oven to 350F.

Line a 9x13-inch baking pan with parchment paper and grease with butter.

Cream butter and sugar in a mixer using a paddle attachment.

Separate eggs and add yolks to the mixer bowl one at a time letting each incorporate before adding another.

Set egg whites aside.

Add Grand Marnier and vanilla to the mixer bowl and blend.

Pulse rusks in a food processor into fine crumbs.

Pulse walnuts in a food processor into walnut meal.

Stir 13 ounces of the walnuts (set the other 3 ounces aside) and rusk crumbs together with the baking powder and spices, and add to the mixer bowl.

In a separate clean and dry mixer bowl, use the whip attachment to beat the egg whites into soft peaks.

Gently fold the egg whites into the mixture, but do not whisk or overmix. Gently.

Spread batter into the parchment lined baking pan.

Bake at 350F for 30 minutes.

While cake bakes, heat the sugar, butter, orange juice and water for the syrup in a saucepan and bring to a boil.

Remove from heat and stir in the vanilla and Grand Marnier.

While cake is warm, dock all over the top with a toothpick.

Ladle some of the syrup over the top then top with the remaining three ounces of walnuts.

Finish ladling the remaining syrup over the top, allowing it to seep in before adding too much.

Let cake cool completely before slicing and serving.