Pre-heat oven to 200°C convention (180°C fan forced/convection).
Place butter and sugar in a small heavy bottomed saucepan on medium heat. Stir and cook for a couple of minutes until butter has melted. Remove from heat.
Place flour and oats in a medium bowl. Pour butter mixture over the flour mixture. Mix well with a wooden spoon.
Grease an 8-inch round shallow baking tin with melted butter. Spoon the flapjack batter into the prepared tin. Spread gently with the back of a metal spoon, pressing down and smoothening the top.
Bake in the pre-heated oven for 16-18 minutes (watch and make sure the slice is golden and the edges are starting to turn a darker brown). Remove from the oven and allow to cool in the tin for 10 minutes (the slice will be very soft when it comes out of the oven but will harden up after 10-15 minutes of cooling at room temperature).
Slice the flapjacks while the batter is still a bit warm. Cool completely and store in an air-tight container in a cool part of your kitchen for up to 5 days.
Notes
Make It Vegan - Use Cocoa Butter instead of regular butter.