Preheat oven to 350°F. Line the bottom of a 10x15 sheet cake pan (cake roll pan) with parchment paper, then spray with nonstick cooking spray.
Whisk both flours, sugar, cocoa, baking soda, and salt in a large bowl. Set aside.
Whisk greek yogurt, almond milk, applesauce, vinegar, and vanilla in a measuring cup. Stir wet ingredients into dry and stir until just blended and smooth.
Pour batter into prepared pan. Bake for 13-16 minutes until a toothpick comes out clean 2” from the edge of the pan. The cake will somewhat spring back when touched.
As soon as the cake comes out of the oven, make the frosting. Place the chocolate chips and almond milk in a medium bowl. Heat in the microwave on high power in 30 second increments, stirring between each, until melted and smooth (about 1 1/2 minutes, depending on your microwave). Spread evenly over the top of the cake. It’ll be a thin layer, but it’ll cover!
Cool completely before serving. Store loosely covered on counter or in refrigerator for up to 3 days.
Serving suggestion: Slice the cake and pipe white laces onto each piece to look like footballs. Use white chocolate chips (melted) or canned white icing for this step, if desired.