Preheat oven to 350 degrees. Butter and flour (or use cocoa powder) an 8×8 cake pan.
Cream butter and sugar together until fluffy. Beat in egg and vanilla. Add cocoa, flour, baking powder and salt and mix until combined. Add yogurt and mix until batter is smooth. Add in milk and mix until batter comes together. Pour into pan.
Bake for 18-20 minutes, or until cake is set. Cake will be thin, and appear a bit spongey. Serve with fresh whipped cream and cocoa syrup.
ETA: some are saying the cake is taking longer to cook – around 25-35 minutes. Just keep an eye on it!
Dark Cocoa Syrup
Mix all ingredients until a smooth glaze forms. If mixture is too thin, add more sugar. If too thick, add more water one drop at a time.