Preheat the oven to 180°C (350°F) and grease a loaf pan with butter or cooking spray.

Melt the chocolate by combining it with water in a small bowl over low heat or in the microwave, stirring until smooth.

Whisk together softened butter and brown sugar in a large mixing bowl until fluffy. Incorporate eggs, vanilla extract, yogurt, and sourdough starter.

Combine the melted chocolate mixture into your batter, whisking until everything is well blended.

Mix the dry ingredients (chai tea latte mix, salt, baking powder) into the all-purpose flour.

Fold the dry mixture into the wet batter gently.

Pour the combined batter into the prepared loaf pan, then bake for approximately 50 minutes.

Cool the loaf cake in the pan for about 10 minutes, then transfer it to a wire rack to cool completely.

Prepare the frosting by adding all frosting ingredients into a food processor or stand mixer, then beat until light and fluffy.

Spread the frosting generously over the cooled loaf cake and sprinkle with chopped dark chocolate if desired.