• Preheat the oven to 160℃ fan / 180˚C/ 355˚F
• Line 2 baking sheets with baking paper.
• In a large bowl, mix the flour, salt, cocoa powder and sugar together.
• Add the softened butter and beat in to the dry ingredients until the mixture forms a smooth paste. It is easier to start off mixing with a spoon and then use your hands.
• It will look very crumbly at first, but it will start to come together.
• If the dough seems very dry after mixing for a minute, add ½ teaspoon of milk.
• Bring the dough together to form a ball.
• Place the dough in between 2 sheets of baking paper, and roll out out to a thickness of 4-5mm (0.15 -0.19 inches)
• Cut into an even number of circles - 5cm / 2¼ inches in diameter.
• Transfer to the prepared baking sheets, leaving a little room inbetween each biscuit as they can spread in the oven.
• Place in the preheated oven and bake for 10 minutes.
• Leave for a minute on the tin, and then move to a wire rack to finish cooling
• When the biscuits are cold, make the lime buttercream.

To Make the Lime Buttercream

• Beat the butter with the lime zest until soft and combined, then sift in the icing sugar. Mix to combine.
• Add 2 teaspoons of lime juice and stir to combine, adding a little more lime juice if the mixture seems too stiff to pipe. However be aware that adding too much lime juice can cause the buttercream to split.
• Spread a teaspoonful of the buttercream - or pipe it - onto half of the biscuits (taking care as they can be crumbly), then top with the other biscuits, twisting slightly to squidge the filling to the edges – not too much or the filling will escape!
• Store in an airtight container.