Prepare the Dough:
In a large mixing bowl, combine the flour, sugar, and salt. In a separate bowl, dissolve the yeast in warm milk and let it sit for 5 minutes until frothy. Add the melted butter, eggs, and vanilla extract to the yeast mixture and whisk to combine.

Mix and Knead the Dough:
Gradually add the wet ingredients to the dry ingredients and mix to form a dough. Turn the dough onto a lightly floured surface and knead for about 8-10 minutes, until smooth and elastic. If the dough is too sticky, add a bit more flour. Place the dough in a lightly greased bowl, cover with a kitchen towel, and let it rise in a warm place for 1.5 to 2 hours, or until it doubles in size.

Make the Raspberry Swirl Filling:
In a small saucepan, combine the raspberries, sugar, and lemon juice. Cook over medium heat for 5-7 minutes, stirring occasionally, until the raspberries break down and become soft. In a small bowl, mix the cornstarch with 1 tablespoon of water to create a slurry. Add this mixture to the raspberries, and cook for another 1-2 minutes until thickened. Remove from heat and let it cool to room temperature.

Shape the Brioche Loaf:
Once the dough has risen, punch it down and transfer it to a floured surface. Roll the dough into a rectangle, about 12×8 inches. Spread the cooled raspberry filling evenly over the dough, leaving a small border along the edges. Starting from one long side, roll the dough tightly into a log.

Place in the Pan:
Grease a 9×5-inch loaf pan. Place the rolled dough into the pan and tuck in the ends. Use a sharp knife to gently score the top of the dough in a few places, allowing it to expand during baking. Cover with a towel and let the dough rise for an additional 30-45 minutes, or until it rises just above the top of the pan.

Preheat the Oven:
Preheat your oven to 350°F (175°C). Bake the loaf for 30-35 minutes, or until the bread is golden brown and sounds hollow when tapped on the bottom.

Cool and Serve:
Remove the brioche from the oven and let it cool in the pan for 10 minutes. Brush the top with melted butter for extra shine and flavor. Remove the loaf from the pan and let it cool completely on a wire rack. Once cooled, dust with powdered sugar if desired and slice to serve.