Butter a 2-1/2 to 3-quart baking dish. (Most 9x13-inch casseroles are 3 quarts.)

Add the cubed brioche to the baking dish in an even layer.

Combine eggs, whole milk, maple syrup, vanilla extract, ground nutmeg and 1/4 tsp of kosher salt until the eggs & nutmeg are well incorporated. (Whisk vigorously, use an immersion blender, or use the low setting on a regular blender.)

Pour the custard slowly and evenly over the brioche. The bread will not be fully submerged. With the back of a spatula or spoon, gently press any dry areas so the bread will make contact with the custard thus encouraging it to begin wicking up the liquid.

Cover the dish and refrigerate overnight (or for at least 2 hours). The custard will fully absorb with time.

Preheat oven to 350°F.

In a small bowl, combine flour, brown sugar and cinnamon with a pinch of kosher salt. Pour in the melted butter and stir with a fork until it resembles wet sand. Dollop bits of streusel over the top of the casserole.

Bake for 45-55 minutes, until it is puffed up and golden brown around the edges. (Timing will vary based on the size/shape of your baking dish and your oven. The internal temperature in the center should be somewhere between 175-190°F. )

While the casserole bakes, make the maple butter: place softened butter in a medium bowl. Add 2 tbsp of maple syrup and a pinch of kosher salt. With a handheld electric mixer, whip together until smooth. Taste and whip in more maple syrup to taste, a teaspoon at a time.