Toss the strawberries with 1/4 cup brown sugar in a bowl. Let the fruit sit for 10 minutes to release some of its juices.Preheat the oven to 350°F. Butter a 10-12 inch baking dish. In a bowl, whisk together the melted butter, remaining 1/2 cup brown sugar, the egg, buttermilk (or milk), and vanilla. Add the flour, baking powder, and salt.Pour the batter into the prepared dish. Spoon the macerated strawberries over the top, then go back and spoon out all the juices. Add small spoonfuls of jam; do not stir. Bake for 30-40 minutes until set around the edges and jammy in the center. Let the cake sit for 5 minutes.Spoon the cake into bowls, getting all the strawberry juices. Serve warm with ice cream or whipped cream.