In a large bowl, combine warm water, brown sugar and yeast, set aside for 5-10 minutes until foamy.

Add flour, pumpkin purée, cinnamon, nutmeg, and salt and mix on low until combined. Increase speed to medium and mix, scraping down the sides of the bowl, until no dry spots remain. This will be a very wet, sticky dough.

Transfer the dough to a large, oiled bowl and cover with plastic wrap. Let rise for 2-3 hours, until doubled in size.

Brush a 9x13-inch pan with 2 Tablespoons melted butter and set aside

Once the dough has risen, transfer the dough to the center of the prepared 9x13-inch pan. Dip your fingers in the melted butter (or you can get the lightly wet), then use your fingers to lightly press the dough until it reaches the edges of the pan

Cover with plastic wrap or a clean kitchen towel and let rise 1 ½-2 hours, until doubled in size and the dough fills (or nearly fills) the pan.

Near the end of the rise, preheat the oven to 450°F and make the topping.

Melt the butter and stir in the brown sugar and cinnamon.

Drizzle the topping evenly over the dough. With wet fingers (you can just dip your fingers in the topping), press deep dimples all over the dough. If you find the dough is sticking to your fingers, you can get them slightly wet, then try again.

Bake for 25-30 minutes, until the focaccia is golden and crusty. Let cool for 10 minutes in the pan, then transfer to a cooling rack to cool fully.

While the focaccia cools, combine confectioners' sugar, milk and vanilla extract in a small bowl. Whisk until smooth and combined. If you want a thinner frosting, add more milk. Drizzle the frosting over the cooled focaccia, slice and serve!