Preheat oven and prep:
Heat oven to 325°F (165°C) using convection. Collect all components.Cream butter and sugars:
Cream softened butter with granulated sugar and vanilla sugar until pale and aerated.Add eggs gradually:
Add eggs one by one, incorporating thoroughly after each addition, until mixture is creamy and voluminous.Mix dry ingredients with milk:
Incorporate flour, baking powder, salt, and 170 ml milk; stir briefly until a homogeneous batter emerges.Divide and cocoa one half:
Split batter into two equal parts. Into one portion, stir in cocoa powder and 2 tbsp milk until just combined.Layer batters in liners:
Drop alternating spoonfuls of plain and chocolate batter into muffin liners.Swirl for marbled effect:
Draw a wooden skewer through the batter to produce a marbled effect.Bake and cool optionally glaze:
Bake in convection oven at 325°F (165°C) for 20-25 minutes. Transfer from pan to wire rack. Optionally, finish with powdered sugar or chocolate glaze.