Preheat your oven to 350F. Grease a 9x13 baking pan. In a large bowl, whisk together the almond flour, granulated erythritol, coconut flour, baking powder, and salt. In a separate bowl, beat the eggs, melted butter, heavy whipping cream, and vanilla extract.Combine the wet and dry ingredients and stir until smooth. Spread into the pan and bake for 18 to 22 minutes until golden and a toothpick comes out clean. Let cool completely, then cut into 1-inch cubes.
Spread the bread cubes on a baking sheet and bake at 300F for 10 to 12 minutes, tossing halfway, until dried out and lightly toasted. This step is important for the bread to properly absorb the custard. Let cool.
Grease a 9x9 baking pan. Arrange the toasted bread cubes evenly in the pan.
In a large bowl, whisk the eggs, heavy whipping cream, almond milk, granulated erythritol, vanilla extract, cinnamon, and nutmeg until smooth. Pour the custard evenly over the bread cubes. Press the cubes down gently with a spatula so they absorb the liquid. Let sit for 15 minutes.
Bake at 350F for 35 to 40 minutes until the custard is set and the top is golden brown. The center should be slightly jiggly but not liquid.
While the bread pudding bakes, whisk together the powdered erythritol, heavy whipping cream, and vanilla extract until a smooth thin glaze forms.
Let the bread pudding cool for 10 minutes, then drizzle the glaze generously over the top. Serve warm.