Brown butter first, cut butter into small pieces in place into a light colored sauce pan if you have one, so that it's easier to see the color of the butter. Melt the butter over medium heat stirring constantly you want the milk solids to separate and burn. The melted butter will start to foam continue stirring it should take about 5 to 8 minutes once butter is a golden amber color, pour into heat proof bowl and set aside.
Prepare apples, use same pan that was used for the butter and toss the apples in along with sugar, syrup, and cinnamon. Stir over medium heat until apples are a bit softened about 4 to 5 minutes.
Preheat oven to 345°F line a 9 x 13" baking dish with parchment paper.
In a medium mixing bowl whisk flour, salt, cinnamon, and allspice. Set aside.
In a stand mixer, with paddle attachment mix together on medium-low speed, brown butter, sugar, eggs, and vanilla extract. Add dry ingredients and mix until combined. Scrape signs with rubber spatula as you go, then with the rubber spatula fold in the apples.
Transfer batter to prepared baking dish and spread evenly within pan. Bake for 30 to 35 minutes until lightly golden. The toothpick shouldn't come out clean. Let cool in pan for about 10 minutes then cut and serve.