Preheat the Oven:
Set the oven to 350°F (175°C).
Line cake pans with parchment paper and grease lightly.
Mix Dry Ingredients:
In a bowl, whisk flour, baking powder, baking soda, and salt.
Cream Butter and Sugar:
Beat butter and sugar until fluffy and pale.
Add Eggs and Vanilla:
Mix in eggs one at a time, followed by vanilla extract.
Alternate Dry and Wet Ingredients:
Add the flour mixture in batches, alternating with sour cream and coffee.
Stir in Hazelnuts:
Fold in finely ground hazelnuts.
Bake the Cake:
Divide batter between pans.
Bake for 30 minutes or until a toothpick comes out clean.
Cool the Layers:
Let the cakes cool completely on a wire rack.
Prepare the Frosting:
Beat butter, powdered sugar, espresso powder, vanilla, and heavy cream until smooth.
Assemble the Cake:
Spread frosting between layers and over the top.
Add the Final Touch:
Sprinkle toasted hazelnuts for crunch.