Can I offer you a piece of my birthday cake? We have way too much to eat ourselves.
Would you like to try a piece of birthday cake? We have lots.
「もしよかったらどうぞ」と丁寧に何かを勧める場合は
Can I offer you ...?
Would you like ...?
が定番な言い方です。(特に Would you like...?)
ーCan I offer you a piece of my birthday cake? We have way too much to eat ourselves.
「もしよかったら誕生日ケーキ(一切れ)いかがですか?自分たちで食べるには多すぎて。」
ーWould you like to try a piece of birthday cake? We have lots.
「誕生日ケーキを一切れいかがですか?たくさんあるので。」
この質問をする前に、
Excuse me. (すみません・失礼します)
と言うと、お食事中や会話中の相手に失礼にならず良いです。
ご参考まで!