Preheat oven to 375°F (190°C) and grease an 8x8-inch baking pan. Whisk milk, egg, vanilla, and sugar until smooth. Stir in the 2 cups Bisquick just until combined.

In a small bowl, combine the 1/3 cup Bisquick, brown sugar, and cinnamon. Cut in cold butter until the mixture resembles coarse crumbs, then stir in nuts if using.

Spread half the batter into the pan, top with half the streusel. Drop remaining batter in spoonfuls on top and spread gently. Scatter the rest of the streusel over it. Optionally swirl with a knife.

Bake for 25 to 30 minutes, until the top is golden and a toothpick comes out with moist crumbs. Tent with foil if browning too quickly.

Cool in the pan for at least 10 minutes, then slice into squares and serve warm.