Free-form Blackberry Gâteau Basque - Dinner With Julie

Ingredients

• 2/3 cup butter, at room temperature
• 1/4 cup packed light brown sugar
• 1/4 cup sugar
• 1 large egg
• 1 tsp vanilla
• 2 cups all-purpose flour
• 1 tsp baking powder
• 1/4 tsp salt
• 1/2-3/4 cup thick cherry (or blackberry) jam
• 1 egg, lightly beaten