In a large mixing bowl, combine softened unsalted butter, powdered sugar, fine salt, and vanilla extract. Mix on medium speed for about 3 minutes until light and fluffy.

Sift in all-purpose flour and Dutch processed cocoa powder into the butter mixture. Mix on low speed until the dough comes together, chilling if too soft.

Preheat oven to 150°C (300°F). Line an 8x8-inch baking tin with baking paper and spray lightly with oil.

Press the dough into the lined baking tin and smooth the top. Bake for 20 minutes. Cut into bars while warm.

Dock the bars and bake again for 20 minutes until golden and firm.

Let cool in the tin for 15 minutes before transferring to a wire rack.

Melt dark cooking chocolate with vegetable oil and dip cooled biscuits for an indulgent coating.