Preheat the oven to 350°F (175°C). Grease a 15 x 10 x 1-inch jelly-roll pan and line it with parchment paper if desired, leaving a little overhang to help lift out the cooled cake.

Combine the devil's food cake mix, melted butter, and egg in a large bowl. Stir until the mixture becomes a thick, cohesive dough with no dry pockets remaining.

Press the dough evenly into the prepared pan. Use lightly greased fingertips or a sheet of parchment to help spread the dough into the corners without tearing it.

Bake for 10–12 minutes, until the center is set and a toothpick comes out clean or with only a few moist crumbs. Do not wait for the cake to become dry, since a slightly soft layer gives the best texture when frozen.

Cool the cake completely in the pan for about 30–45 minutes. The cake must be fully cool before the ice cream is added, or the ice cream may melt too quickly.

Cut the cooled cake in half widthwise to create two equal rectangles. Work carefully with a serrated knife and use the parchment, if present, to help transfer or reposition the cake.

Spread the softened vanilla ice cream evenly over one cake rectangle. It should be pliable enough to spread but still firm rather than melted.

Place the second cake rectangle on top of the ice cream and press gently to level the sandwich. Cover the pan tightly and freeze for at least 1 hour, or until the slab is firm.

Cut the frozen slab into approximately 15 bars. For clean cuts, warm a large knife under hot water, wipe it dry, slice, and repeat between cuts.