Preheat the oven to 180 degrees C. Grease and line a 20cm square baking tin then set aside.
Mix the melted butter, flour and a 1/2 cup of the brown sugar in a bowl until combined then press the mixture evenly into the base of the baking tin. Bake in the oven for 15-20 minutes or until starting to brown on top. Remove from the heat and set aside to cool.
For the caramel, place the condensed milk, unsalted butter, sea salt flakes and remaining 1/2 cup of brown sugar in a saucepan. Cook over low heat until melted and well combined, stirring constantly to prevent it from burning and until it darkens to a caramel colour. Remove from the heat and stir through the lavender oil until well combined.
Pour the caramel over the cooled base then bake for a further 10-12 minutes or until the caramel starts to bubble and turn golden. Remove from the oven and set aside to cool.
For the chocolate ganache topping, place the cream and chocolate in a heatproof bowl over a saucepan of simmering water. Use a whisk and slowly mix until glossy and smooth. Spread the ganache over the top of the cooled slice then refrigerate for one hour or until set. Scatter with dried lavender and a pinch of sea salt flakes if using. Use a hot, sharp knife to cut into squares before serving.
If sea salt is not available in your pantry, try using salted butter for the caramel filling!