1Preheat oven to 300 degrees F (150 degrees C). Grease a 9-inch square baking pan and line with parchment paper.
2Stir together butter, sugar, 1/2 cup plus 1 tablespoon golden syrup, and molasses in a saucepan and cook over low heat, stirring constantly, until butter is melted and mixture is smooth, 4 to 5 minutes. Remove from heat and let mixture cool 5 minutes.
3Whisk flour, baking soda, and oats together in a large bowl. Stir in butter mixture. Add egg, then milk, and stir until well combined. Pour batter into prepared pan and spread evenly.
4Bake until a toothpick inserted into center comes out clean, about 1 hour 20 minutes. Remove from oven and let cool completely in pan, about 1 1/2 hours.
5Cover pan tightly with foil or plastic wrap, without letting it touch surface of cake, and let stand at room temperature for at least 1 day and up to 5. Cut into squares and serve.