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I'll settle for just one cake.
Literal
Cake one-piece [with-で] bargain [object-を] strike [for-you-てあげるよ].
手を打つ (てをうつ, 'to strike a deal / to come to terms') is an idiom from bargaining — literally 'to clap hands,' referencing the traditional gesture of sealing a deal. ケーキ一個で ('with just one cake') sets the bargaining price using the counter 個 and instrumental で. ~てあげるよ ('I'll do it for you') adds a condescending generosity — the speaker is conceding to a lower demand as a favor. Playful, possibly between children or in a lighthearted negotiation.