Pre-heat the oven to 200C/392F.
Melt the butter in a small saucepan over a low heat.
Working a sheet at a time, halve each sheet and layer each half, brushed with butter to create a stack of four half sheets. Brush the insides of 9 holes of the muffin tin with butter and press each stack inside, creating a little cup - you may need to squish some of the sides down depending on the size of your pastry sheets. Don’t brush the top layer or inside of the cups with butter as this will make your cups greasy!
Using a fork, pierce clear holes in the bottom of each cup to stop them puffing up in the oven - but don’t worry if this happens, while they’re still warm you can press the middles down again with a dessert spoon.
Bake the cups for 8-10 minutes until golden. The moment they’re cool enough to touch, transfer them to a wire rack lined with kitchen paper to soak up any additional butter to cool.