Preheat oven to 350 degrees F. Grease a 15x10x1-inch baking pan. Line with parchment paper. Grease and flour the paper; set pan aside.

In a small saucepan, place 3/4 cup of the dark chocolate baking chips with the shortening. Heat over low heat until melted and smooth; set aside to cool.

In a medium bowl, stir together the flour, cocoa, baking soda, and 1/2 teaspoon salt; set aside.

In a large bowl, beat 3/4 cup butter with an electric mixer on medium speed for 30 seconds. Add granulated sugar and beat 2 minutes more or until fluffy. Add eggs, one at a time, beating on low after each. Beat in chocolate mixture and vanilla. Alternately add flour mixture and buttermilk into butter mixture, beating on low after each addition. Spread batter into the prepared pan.

Bake about 25 minutes or until a toothpick inserted into the center comes out clean. Cool in the pan on a wire rack.

Meanwhile for the frosting: Melt the remaining 1 cup dark chocolate baking chips and cool. In a very large mixing bowl, beat 3/4 cup butter with an electric mixer on medium speed for 30 seconds. Beat in melted chocolate, 1/2 teaspoon salt, and 1 cup of the powdered sugar until combined. Beat in milk and vanilla. Gradually beat in the remaining powdered sugar. If necessary, beat in enough additional milk until smooth and spreadable.

Cut rounds of the cake using a 3-inch round cutter or heart cutter.* Halve the cakes crosswise to form 2 thin layers. Spread frosting over each round (both layers). Stack one frosted round over the other. Sprinkle with mini baking chips.