Preheat oven to 350°F. Line an 8x8-inch baking pan with foil, parchment paper, or grease and flour pan. Set aside.
Prepare brownies. In a bowl mix together melted chocolate and oil until smooth. Beat in eggs and sugar until well combined. Fold in flour, baking powder and salt until just mixed together.
2 ounce unsweetened chocolate, 1/3 cup vegetable oil, 2 whole large eggs, 1 cup granulated sugar, 3/4 cup all-purpose flour, 1/2 teaspoon baking powder, 1/2 teaspoon salt
Pour into prepared pan. Bake for 20-25 minutes until a tester comes out mainly clean. Remove from oven and cool completely.
Once brownies are completely cool prepare the chocolate layer. In a large bowl using an electric mixer or a stand mixer, beat butter and sugar until light and fluffy; about 2 minutes.
1/2 cup unsalted butter, 1/2 cup granulated sugar
Gradually mix in cooled chocolate. Add vanilla and mix until well combined.