I have several cupcake recipes. I know it's easy to convert to a cake (just adjust baking time and figure out correct pan size). My question is many of the cupcake recipes have a filling that is (for the most part) just sort of stabbed into the baked cupcake using a squeeze bottle. These fillings are usually between artificial maple syrup and pudding in consistency/thickness. If I want to use the filling, how would I incorporate it into the cake other than repeatedly "stabbing" the cake in various places? A google search only tells me to tort the cake, but I'd like to leave it in a single layer if possible (plus I don't think the cupcake fillings would hold up to be filling layer in a layered cake).
A syringe (or baster, but a syringe will be better) A LOT of holes, and frosting to hide the holes (though they will be pretty small if you use a syringe.)
Given the comment that "I don't want it to be like several filled donuts joined together where it's not consistent" in combination with a refusal to consider layering it, you need
- a dense pattern of holes and
- a measured amount of filling in each hole.
So, for a 9x13 pan, you might be looking at 8x12 = 96 holes an inch apart, starting 1/2 inch from the edge, each filled with 1/96th of your total amount of filling. Adjust as needed for other size cakes and how well an inch apart suits your idea of "consistent" - you could go to 17x25 holes 1/2 inch apart and 1/4 inch from the edge, but it's going to be rather tedious.
Or you could TRY a more cake-centric approach - half the batter, the filling, half the batter, bake. It might fail miserably, it might "work" for some value of "work," it might require changing the filling to get a better value of "work." Certainly marble cakes and pudding cakes do things of this nature at the batter stage before baking.
Even in industrial baking, pastries are typically filled by poking a hole in the pastry and filling them by hand. At least this was my experience with some equipment design provisions my company made for a prominent Midwestern U.S. donut factory. Your best bet is likely finding a way to conceal the hole rather than avoiding it.