1️⃣ Preheat and prepare the pan:
Preheat your oven to 350°F (175°C). Grease a 9-inch square baking pan thoroughly to prevent sticking.

2️⃣ Make the cake batter:
In a medium mixing bowl, whisk together the flour, granulated sugar, cocoa powder, baking powder, and salt. Gradually stir in the milk, melted butter, and vanilla extract until the batter is smooth. Pour the batter into the prepared baking pan, spreading it evenly.

3️⃣ Prepare the fudge sauce:
In a separate bowl, mix the granulated sugar, brown sugar, and cocoa powder. Sprinkle this mixture evenly over the cake batter in the pan.

4️⃣ Add the boiling water:
Carefully pour the boiling water over the top of the batter and sugar mixture. Do not stir! The water will create the gooey fudge sauce as the cake bakes.

5️⃣ Bake the cake:
Place the pan in the oven and bake for about 35 minutes, or until the top of the cake looks set and the edges are firm. The center may jiggle slightly due to the sauce beneath.

6️⃣ Cool and serve:
Allow the cake to cool for about 15 minutes. Serve warm, spooning the rich fudge sauce from the bottom of the pan over each portion. Pair with vanilla ice cream or whipped cream for an extra treat.