Preheat oven to 355ºF | 180ºC and line a rectangular cake pan with parchment paper.
In a bowl, beat the yolks and eggs with the sugar for 15-20 minutes (speed 4) until you get a strong and bulky eggnog. The dough will thicken and turn light yellow and will be ready when it doesn’t drain from the mixer blades.
With the mixer at a minimumm(speed 1) or with a spatula, slowly add the flour until completely blended in the dough.
Pour the batter into the pan and bake for approximately 15-20 minutes. The sponge cake should be moist and slightly undercooked.
In a small saucepan, place the blueberries and cook over low heat for 5-10 minutes, stirring frequently. Transfer to a food processor or blender and grind until you get jam. Set aside.
In the same saucepan (don't worry if it stains), add the sugar and water and let it boil for 5 minutes. Remove from heat and stir with a wooden spoon until the sugar is translucent. Add the blueberry jam and mix well.
Remove the sponge cake from the oven and cover it immediately with the icing: with a spoon or a brush, spread the syrup over the cake making back and forth movements until the gaze is opaque.
Let dry, unmold and top with blueberries.