In a small saucepan, combine the dark brown sugar, milk, water, and salted butter. Stir over medium heat until the sugar dissolves and the mixture is smooth. Remove from heat and set aside.
While the syrup cools, preheat your waffle maker according to the manufacturer's instructions.
In a large mixing bowl, whisk together the full-fat milk, water, and eggs. Melt the stick of butter and add it to the wet mixture, whisking until well combined.
In another bowl, whisk together the plain flour, sugar, baking powder, and salt.
Pour the dry ingredients into the wet ingredients and gently mix until just combined. A few lumps are fine.
Lightly grease the waffle maker if necessary. Pour the batter into the preheated waffle maker and cook according to your waffle maker's instructions, usually about 4-5 minutes or until golden brown.
Once the waffles are cooked, carefully remove them and drizzle with the warm brown sugar butter syrup.