Heat the oven to 350 degrees Fahrenheit and lightly butter the sides of a 9x2-inch round cake pan.
Make the caramel-nut topping: In a small saucepan, combine the brown sugar, butter, and 3 tablespoons water.
Cook over medium heat, stirring often, until the butter is melted and the mixture is smooth.
Bring to a boil and pour into the prepared pan, swirling to coat the bottom evenly.
Scatter in the nuts evenly and gently press them in.
Make the cake: Sift together the flour, cocoa powder, baking powder, baking soda, and salt.
In a medium bowl, beat the butter with an electric mixer until smooth. Gradually add the sugar and continue beating until fluffy. Beat in the vanilla.
Add the eggs one at a time, beating briefly after each addition.
Sprinkle half of the flour mixture over the butter mixture and mix on low speed just until the flour disappears.
Add the buttermilk and mix until just blended. Gently mix in the remaining flour.
Scoop spoonfuls of batter onto the nuts and gently spread the batter evenly in the pan. Lightly tap the pan on the counter to settle the ingredients.
Bake until a toothpick inserted in the center comes out clean, about 45 minutes.
Immediately run a paring knife around the inside edge of the pan.
Set a flat serving plate on top of the pan and invert the cake. Let the inverted pan rest for about 3 minutes to let the topping settle.
Gently remove the pan and serve slightly warm or at room temperature.