Step 1: To prepare the blueberry compote: In a medium saucepan, combine the blueberries, lemon zest, lemon juice and sugar. Bring to a boil and reduce heat to low and simmer. Stir the ingredients occasionally and press berries to release juices. Some of the berries can be left whole. Stir for roughly 5 minutes or until the sauce has thickened.
Step 2: Place the slices of Brie on top of 4 waffles. Top each waffle with a generous scoop of prepared blueberry compote. Place the remaining 4 waffles on top to make 4 sandwiches.
Step 3: Heat butter in a frying pan over medium heat. Place sandwiches in the pan and grill, flipping once, until waffle is golden and cheese has melted. This process takes roughly 2-3 minutes per side. You may need to reduce heat to melt the cheese without burning the waffle. Serve warm.