Preheat oven to 335F
Using a food processor, process the hazelnuts and potato starch until finley ground.
Move the ground hazelnuts into a medium bowl, then mix in the flour, baking powder, baking soda, nutmeg, and orange zest, set aside
In a bowl of a standing mixer, fitted with the paddle attachment, beat the butter, sugar, and honey on medium speed for 4-5 minutes or until light and fluffy.
Increase speed to medium-high and add the vanilla extract followed by the eggs, one at a time waiting for the first egg to fully incorporate before adding the second. Scrape the sides and bottom of the bowl.
Reduce speed to low, and add the hazelnuts-flour mixture in three additions altering with the sour cream starting and end ending with the dry mixture. Add the last portion of the flour mixture using a rubber spatula.
Scoop the cake batter into the prepared pan over the syrup and hazelnuts, then use an offset spatula to evenly spread it.
Bake for 35-40 minutes or until a toothpick comes out clean at the center of the cake.
Remove, allow to cool for 2-3 minutes then invert the cake onto your serving plate, or an 8-inch cake board.