Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, and line a few baking trays (or however many you own) with baking paper
Mix the butter and golden caster sugar together in a large bowl using an electric whisk, or silicone spoon
Add the egg, lemon zest and vanilla extract, and mix them in with a spoon or spatula
Mix in the oats, plain flour and baking powder with a spoon
Spoon blobs of the mixture onto your lined baking trays, then add 3 or 4 blueberries per cookie. Push them into the batter and cover them with batter. Leave space between the cookies as they will spread
Bake the cookies for 12-14 minutes until they are golden brown, leave them on the tray for 5 minutes before carefully removing and placing on racks to cool fully
Make the icing by mixing the lemon juice and icing sugar together into a thick paste. Add the lemon juice a little at a time as you may not need it all. Once the cookies are fully cool drizzle it all over them
The cookies are best served on the same day. They are still good for 2 days after, but they do go softer and more fragile because of the moisture from the blueberries