In a mixing bowl, whisk the melted butter and brown sugar until smooth and combined.
Add the egg and vanilla extract, then whisk again until the mixture looks uniform.
Stir in the flour, baking powder, and salt until no dry streaks remain.
Line the slow cooker with parchment paper and pour in the batter, spreading it evenly.
Drizzle the caramel sauce over the surface and gently swirl it through the batter with a knife.
Cover and cook on low for 2 to 3 hours, until the edges look set but the center remains slightly soft.
Sprinkle flaky sea salt over the top while the blondies are still warm, then lift them out using the parchment to cool slightly before slicing.