Preheat oven to 350°F (175°C); line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides and lightly greasing the paper.

Melt ½ cup butter in a small saucepan over medium heat; add brown sugar and heavy cream, bringing to a gentle simmer while stirring until smooth (2-3 minutes), then remove from heat and stir in vanilla extract and a pinch of sea salt, setting aside to cool.

Whisk together 1 cup melted butter and granulated sugar in a large bowl. Beat in eggs one at a time, then stir in 1 teaspoon vanilla extract until smooth.

In a separate bowl, whisk together flour, cocoa powder, baking powder, and ¼ teaspoon salt.

Gradually add dry ingredients to wet ingredients, mixing until just combined without overmixing.

Pour two-thirds of the brownie batter into the prepared pan and spread evenly; drizzle half of the cooled butterscotch over the batter in stripes, then drop spoonfuls of remaining brownie batter and drizzle the remaining butterscotch on top.

Use a knife or skewer to gently swirl the butterscotch and brownie batter together for a marbled effect, taking care not to overmix.

Sprinkle flaky sea salt over the brownies and bake for 25-30 minutes until a toothpick inserted into the center comes out with moist crumbs and the edges are set but the center remains gooey.

Remove from oven and let cool completely in the pan on a wire rack before lifting and cutting into squares to ensure a fudgy texture.