Preheat the oven to 180C. Grease and line the bottom and sides of an 8" / 20cm x 20cm square baking tin with greaseproof / parchment paper.
In a large saucepan over a low heat, place the unsalted butter, peanut butter, and honey. Stirring constantly, allow these to melt and infuse together. Remove from the heat.
Stir in the oats, salt and vanilla extract and mix well together.
Scrape the contents of the saucepan into the baking tin and press down all over with the back of a spoon ensuring an even covering.
Bake for 20-25 minutes until golden brown.
Once cooked remove from oven and leave in the tin placed on a wire rack to cool completely.
Once cooled carefully remove the oat slab from the tin, place on a wooden board and cut into 9 squares.
The flapjacks will keep in an airtight tin for up to 10 days.