In a large saucepan, heat the butter over medium heat, stirring occasionally, until melted and it turns toasty and brown, about 5 minutes.
Add the butter to the bowl with the pumpkin and stir to combine. Add the eggs, spice, malted milk powder, and maple extract. Stir to combine. (This will be too thick for a whisk. I start with a fork and then move to a rubber spatula.)
In the same large saucepan, whisk together the golden syrup, brown sugar, heavy cream, and salt. Have an instant-read thermometer ready.
Bring to a boil over high heat. Boil, whisking constantly, until the thermometer registers 245°F (118°C), about 2 minutes.
Stir the hot syrup into the pumpkin mixture, in two batches. Spread the filling into the crust.
Return to the oven and bake until set and the surface is matte and dry to the touch, about 30 minutes.
Transfer the tart tin to a wire rack and let cool for at least 20 minutes. Serve slightly warm or at room temperature.