Place the fresh blackberries, granulated sugar, and lemon juice in a small pot over low to medium heat.
Using a potato masher, gently crush the blackberries until the berries release their juices and form a thin sauce.
Increase the heat to medium-high.
Allow the blackberry mixture to bubble and reduce, stirring frequently, until thickened into a jam-like consistency.
Remove the blackberry jam from the heat and allow it to cool slightly. The mixture will continue to thicken as it cools.
Butter one side of each slice of bread.
Place two slices of bread, butter-side down, in a large pan with the heat turned off. Work in batches if all sandwiches do not fit in the pan at once.
Spread approximately 1 heaped tbsp of blackberry jam over each slice of bread in the pan.
Top each slice with approximately 3 wedges of brie.
Add 2 slices of cooked bacon to each sandwich.
Sprinkle each sandwich with a pinch of chilli flakes, if desired.
Place another slice of bread on top of each sandwich.
Butter the outside of the top slice of bread.
Turn the heat to medium.
Cook the sandwiches until the bottom slices are golden brown and crisp.
Carefully turn each sandwich over.
Continue cooking until the second side is golden brown and the brie is melted.
Adjust the heat as needed to prevent the bread from browning too quickly before the cheese melts.
Remove the sandwiches from the pan.
Repeat the cooking process with the remaining sandwiches, if needed.
Slice each sandwich in half.
Serve immediately while hot, crisp, and melted.