This is not exactly what you are looking for and this is from scratch, and yes, you would have time to make this as it goes together quickly.
These are rich and delicious and similar to Texas Sheet Cake.

Iowa Brownies

1 stick butter (ONLY!), softened
1 cup sugar
1 (16 ounces) Hershey's Syrup
4 eggs
1 cup flour
1 cup chopped walnuts (optional)
Icing (recipe follows)

Cream butter and sugar. Beat in syrup and eggs until well mixed. Beat in flour. Add nuts, if used. Pour into greased 15"x10"x1" jelly roll pan. Bake 25 minutes at 350 degrees F. DO NOT OVER-BAKE. Cool. Ice.

Icing

1 stick butter (only!)
1 1/2 cups sugar
1/3 cup evaporated milk
1/2 cup chocolate semi-sweet chocolate chips

Place butter, sugar, and milk in a 2 quart sauce pan and melt the butter, stirring well. Bring to a boil and boil one minute; remove from heat. Stir in chocolate chips until melted. Pour over cooled cake.

This can be cake or brownies

1/2 c. butter
1 c. sugar
3 eggs
Dash of salt
1 c. flour
Pecans or walnuts for garnish
3/4 c. chocolate flavored syrup canned
2 tsp. vanilla
3/4 c. chopped walnuts or pecans

Cream together butter, sugar, and eggs until very creamy and well blended. Add salt. Stir in flour, mix well to blend. Add chocolate syrup, vanilla, and nuts. Turn mixture into well greased and lightly floured 9 inch square pan. Smooth top. Bake at 350 degrees for about 35 minutes or until toothpick comes out clean. Cool in pan but loosen edges with knife. Cut in squares and garnish with nuts, or dust with powdered sugar.

You can do that with any cake! just like you said poke holes & drizzle with the syrup!