1/2 cup pecans, chopped
For the topping:
1/4 cup (1/2 stick) unsalted butter
1/4 cup packed dark brown sugar
2 medium carrots
Preheat the oven to 350°F. Butter a 9-inch-round cake pan. Line it with parchment and butter the parchment. To prepare the cake, whisk together the flour, baking powder, cinnamon, nutmeg, ginger, cloves, and salt in a medium bowl. In a large bowl, beat the 1/2 cup of butter and the 2/3 cup of brown sugar with an electric mixer on medium speed until pale and fluffy, 3 to 4 minutes. Beat in eggs and yolk, one at a time, until combined, and then stir in the vanilla. With the mixer on low, alternate adding the flour mixture and buttermilk to the butter mixture, starting with the flour. Set the batter aside while you finish the topping.
To make the topping, melt the 1/4 cup butter in a skillet over medium heat. Add the 1/4 cup brown sugar, stirring constantly, until the mixture bubbles and starts to take on the texture of caramel, about 2 minutes. Remove from the heat and transfer the mixture to the prepared pan.