Line an 8×8-inch pan with parchment paper, leaving overhang. Preheat oven to 350°F (175°C).
In a bowl, stir melted butter and brown sugar. Add flour and desiccated coconut; mix until a thick, crumbly dough forms.
Press the base firmly into the lined pan. Bake 18–22 minutes until lightly golden.
For the caramel, melt butter in a saucepan over medium heat. Stir in brown sugar.
Stir in vanilla and sweetened condensed milk. Cook, stirring constantly, until thick, smooth, and glossy (8–12 minutes).
Pour caramel over the baked base and spread evenly. Chill 1–2 hours until firm.
Melt chocolate and stir in vegetable oil until smooth.
Pour chocolate over the set caramel and spread into an even layer.
Chill 45–60 minutes (or longer) until the chocolate is firm.
Lift out using parchment, slice into squares with a warm, dry knife, and serve.