Place an 8-inch cake on a cake cardboard and place it on a cake turntable. Place a big dollop of green frosting on the cake layer—about 2/3 cup—and use a metal spatula to spread the frosting into a layer going all the way out to the edges of the cake. Top with a second green cake round. Spread a thin layer of frosting around the sides, to lock in any crumbs and provide some stability.
Repeat this process with the other 2 cake layers. At this point you should have 2 cakes, each consisting of two layers. Refrigerate the cakes for an hour, or freeze them for 30 minutes, until they are very firm.
While you wait for the cakes to chill, prepare the red filling. Crumble the red cake into a large bowl, working it between your fingers until it is in fine crumbs. Add about half of the red frosting, and stir them together. Gradually add more red frosting, adding just enough so that the cake is moist and holds together when you press it into a ball. Don’t add so much that it becomes goopy or greasy.
When the green cakes are very firm, leave 1 in the refrigerator while you work on the other. Make the top of the heart first. Use a large round 5” cookie cutter (or a 5” bowl) to make an indent in the center of your cake to help you know where to cut.
For the top, you want to first think about carving out a “W” shape—the top of the heart needs to have a peak in the center, sticking up. Take a sharp paring knife and start cutting around the circle indent you made, angling the knife down and inward about halfway to the center of the cake. Once you’ve gone all the way around the circle, place your knife in the center of the cake, this time angling it outward as you cut around, trying to get the angle right so that it meets your previous cut. Once you’ve gone all the way around, pop out the piece of cake you cut. You should have a crude “W” shape with a cone of cake in the middle.
Use a spoon to round out the cake’s cavity, scooping out cake from the center as necessary. Trim the cone in the middle so it has a smooth, organic shape to it rather than being a jagged point. Think about how smooth and round the top of a heart is, and try to recreate that shape.
Once you have scooped, trimmed, and smoothed, take the red cake mixture and press it into the cavity. Fill the hole completely, then smooth the top of the cake so the red cake is completely level with the green cake. Refrigerate this layer while you work on the second one.
Good news--the bottom of the heart is easier! Mark the same 5-inch circle on top, and then cut around it. As you cut, angle your knife down toward the center of the cake, so you cut out a cone shape. Pop out the cone piece, and do any necessary trimming to make the bottom of the heart smooth and even. Fill the cavity with red cake and smooth the top.
Spread a thin layer of green frosting around the edge of the cake. Take the second cake, the one with the top of the heart, and flip it over onto the bottom layer. Spread a thin layer of frosting all along the sides and top of the cake, then refrigerate it until completely firm, at least 45 minutes.
To finish the cake, spread a thicker layer of frosting on the top and sides of the cake, and smooth it out with a spatula or bench scraper. Press the fondant eyes into the sides of the cake. Draw on a Grinch face with the black frosting. (I used a reference picture I found online and sketched out the face first, making it the approximate size it would be on the cake—this helped me translate it from the page to the cake surface.) Finally, press the fondant hair into the top of the cake.
For the cleanest cuts, cut this cake when it is fairly cold, but for the best taste and texture, enjoy the slices at room temperature.