1 cup butter, softened
1 cup sugar
2 egg yolks
1/4 cup vegetable oil
1/2 cup cocoa powder
2 teaspoons vanilla
2-1/4 cups all-purpose flour
1/4 teaspoon salt
4 ounces semisweet or bittersweet chocolate, chopped into chunks
2 Tablespoons heavy cream
Cream the butter and sugar together in a large mixing bowl until light and fluffy. Blend in the egg yolks.
In a small bowl, whisk together oil, cocoa and vanilla until smooth. Add cocoa mixture to the butter mixture and stir until completely incorporated, scraping the sides of the bowl every now and then.
Add the flour and salt to the wet ingredients, blending with a mixer until combined. Refrigerate cookie dough for 30 minutes to firm up.
Preheat oven to 350 degrees. Scoop out balls of cookie dough with a small ice cream scoop and place 1 inch apart on parchment-lined cookie sheets. With thumb or handle of wooden spoon, make small indentation in center of each ball.
Bake for 12 to 15 minutes, until a toothpick inserted in the cookie center comes out clean. Let cookies cool completely.
To make the chocolate ganache, slowly melt chopped semisweet chocolate in the microwave. Add cream and whisk vigorously until smooth and thick. Alternatively, melt chocolate over a double boiler. Place ganache in a piping bag and squeeze onto the indentation in the cooled cookies. Leftover chocolate ganache can be used to drizzle across the outer edges of the cookies. Let chocolate set completely before serving.