Use Base Tree from XGBoost
There are 2 solutions for you:

Build only 1 tree (meaning use only 1 iteration), that means that when you train use num_boost_round=1
When you predict use ntree_limit=1

Either one will solve your problem. You either train only 1 tree or you predict using only 1 tree.
Is there a way to only use the base decision tree that is used in the XGBoost algorithm?
I know that Sklearn's GBT just uses a Sklearn Decision Tree as their base but XGBoost builds trees differently (e.g. regularization of the leaf weights).
I looked at the code for XGBoost but I wasn't able to figure out how they built their base tree.
Thanks!