Preheat your oven to 350°F (175°C) and generously grease a 10-inch bundt pan with softened unsalted butter. Dust with flour and shake out the excess.

In a medium bowl, whisk together all-purpose flour, ground hazelnuts, baking powder, baking soda, and salt until well blended. Set aside.

In a large mixing bowl, beat softened unsalted butter and granulated sugar on medium-high speed for 3-4 minutes until light and fluffy.

Add eggs one at a time, mixing well after each addition, followed by vanilla extract.

Gradually add the dry mixture to the batter in three parts, alternating with sour cream and brewed espresso, mixing gently till combined.

Spoon the batter into the greased bundt pan, smooth the top, and gently tap the pan on the counter.

Bake in the oven for 45-55 minutes until a toothpick comes out clean or with a few moist crumbs.

Remove and cool for 15 minutes on a wire rack before inverting onto the wire rack to cool completely.

For the glaze, heat heavy cream with butter until simmering, then pour over finely chopped dark chocolate. Stir until smooth.

Pour the glaze over the cooled cake and sprinkle toasted hazelnuts on top before serving.