Place the oatcakes in a food processor and whiz for a minute or two to form crumbs.
In a bowl, mix the oatcake crumbs together with the melted butter, then tip into the cake tin. Use your fingers to press down and form a base.
In a seperate bowl, use a whisk to mix together the condensed milk, egg yolks, lime juice and lime zest. Then scrape the mixture into the cake tin. Place into the oven and cook for about 20 minutes until just set but still a little wobbly.
Place on a rack to cool, before placing in the fridge to cool completely where it will continue to set.
Run a sharp knife around the edge before removing from the tin completely, cutting into slices and serving.
6 Nairn's Rough oatcakes
50g butter, melted
1x 397g tin of sweetened condensed milk
5 egg yolks
150ml lime juice (about 4 or 5 limes)
Finely grated zest of 1 lime