Preheat your oven to 425°F (220°C).
In a large mixing bowl, whisk together the milk, eggs, vanilla extract, and granulated sugar until well combined.
In a separate bowl, sift together the all-purpose flour, unsweetened cocoa powder, and coarse salt.
Gradually add the dry mixture to the wet ingredients, whisking until just combined.
In your oven-safe skillet, add the unsalted butter and place it in the preheated oven for about 2 minutes, or until melted and bubbly.
Remove the skillet from the oven and carefully pour the batter over the melted butter.
Return the skillet to the oven and bake for 20-25 minutes, or until the edges are puffed and the center is set.
Dust with powdered sugar and serve immediately with fresh strawberries, if desired.