Line the baking pan with parchment paper or grease it with butter and sprinkle with flour or breadcrumbs.

Preheat the oven to 170 ℃ (fan-assisted) or 180 ℃ (top/bottom heat mode).

Transfer the eggs and sugar to a bowl - you can also add a little vanilla extract if you like.

Beat the eggs and sugar together into a thick and fluffy mixture - I beat for several minutes on the highest speed of the mixer.

Mix the wheat flour with baking powder.

Add the flour in portions to the beaten eggs, mixing only on low speed of the mixer.

Add the flour alternately with buttermilk.

Add oil and mix briefly - just until combined.

Do formy przełóż mniej więcej połowę ciasta i rozprowadź równą warstwą.

Spread the blackcurrant jam on the dough and gently spread it.

Spread the rest of the dough and gently spread it.

Transfer the streusel ingredients to a food processor or a large bowl and briefly mix or crumble together until crumbly.

Sprinkle the streusel over the top of the cake.

Place the pan with the cake into the preheated oven and bake for about 40-50 minutes - until a skewer inserted comes out clean.

You can serve the cake after it has slightly cooled.