Preheat the oven to 180C (350F).

Finely grind the hazelnuts in a food processor until they reach a flour-like consistency.

In a large bowl, combine the barley flour, sugar, ground hazelnuts, and baking powder.

In a separate bowl, whisk together the eggs, yogurt, vegetable oil, and vanilla extract or seeds.

Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.

Grease and flour a bundt cake pan to prevent sticking.

Pour the batter into the prepared bundt cake pan.

Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center comes out clean.

Remove the cake from the oven and let it cool in the pan for 10-15 minutes before inverting it onto a wire rack to cool completely.