What is the fine-grained topic of the following text: Stylistically a Baltic Porter, this opaque black lager is brewed with a blend of Pilsner and Maris Otter base malts, while the dark color and roasty flavor comes from both caramel and chocolate malts. Only return the topic.
Brewing a Baltic Porter with a blend of Pilsner and Maris Otter base malts, caramel and chocolate malts.