Preheat the oven to 170 °C (338°F) on the static setting.
In a bowl, mix the buckwheat flour, hazelnut flour, baking powder, and salt. Set aside.
In another bowl, beat the whole eggs with the sugar using an electric whisk for 4 minutes or until the mixture is fluffy.
Then, without turning off the electric whisk, slowly add the olive oil to the mixture.
Finally, add the flour, then milk. Mix the ingredients with an electric whisk for 2 minutes.
Line a baking pan with parchment paper, pour in the mixture, and bake in a preheated oven for about 45 minutes. Alternatively, insert a toothpick into the center of the cake during the last few minutes of baking. If it comes out dry and clean, the cake is ready. If there are any moist batter or crumbs on the toothpick, continue baking for another 5 minutes.
Allow to cool completely before removing the cake from the mold and cutting it in half to fill with jam.
Dust with powdered sugar.