In a bowl, beat butter, oil, and brown sugar until fluffy.

Add eggs and continue beating until well combined.

In a separate bowl, mix flour, baking powder, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Incorporate Greek yogurt into the batter.

Add vanilla extract if desired.

Divide the dough into two separate bowls.

Melt nougat in milk using a microwave (approximately 45 seconds at 700W).

To one half of the dough, add dulce de leche and mix well.

To the other half of the dough, add cocoa, ground hazelnuts, and melted nougat. Mix well.

Preheat oven to 175C (350F).

Spoon the dark (nougat) dough into muffin tins or molds.

Top with the light (dulce de leche) dough, or vice versa.

Bake at 175C (350F) for approximately 25 minutes.

If the muffins start to darken too quickly, cover them with aluminum foil during the last part of baking.

Let cool slightly before serving and enjoy!